Issue during Subcontracting -> Purchase Receipt: NoneType object is not iterable

Hi all,
I was following the tutorial video (https://frappe.github.io/erpnext/user/manual/en/manufacturing/subcontracting) on how to create a subcontract. Everything is working fine but for the final step - the material receipt. It can not be submitted and throws an Error: ‘NoneType’ object is not iterable. Please see below for complete error log.
At least two other people seem to face the exact same issue:
TypeError: ‘NoneType’ object is not iterable - While creating a Purchase Receipt
https://github.com/frappe/erpnext/issues/9842

I have followed the advise given on these two posts and updated the system, checked that all warehouses have accounts assigned to them and verified that the suppliers have defined currencies. However, the problem still exists (regardless of which warehouse/supplier/BOM I choose to do the subcontracting with).

Any help would be highly appreciated!
Best regards
Timo

Error Log:
Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/desk/form/save.py”, line 19, in savedocs
doc.submit()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 741, in submit
self._submit()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 730, in _submit
self.save()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 230, in save
return self._save(*args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 280, in _save
self.run_post_save_methods()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 793, in run_post_save_methods
self.run_method(“on_submit”)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 666, in run_method
out = Document.hook(fn)(self, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 887, in composer
return composed(self, method, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 870, in runner
add_to_return_value(self, fn(self, *args, **kwargs))
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 660, in
fn = lambda self, args, **kwargs: getattr(self, method)(args, **kwargs)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py”, line 135, in on_submit
self.make_gl_entries()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/controllers/stock_controller.py”, line 30, in make_gl_entries
make_gl_entries(gl_entries, from_repost=from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 19, in make_gl_entries
save_entries(gl_map, adv_adj, update_outstanding, from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 88, in save_entries
make_entry(entry, adv_adj, update_outstanding, from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 99, in make_entry
gle.insert()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 192, in insert
self.run_before_save_methods()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 772, in run_before_save_methods
self.run_method(“validate”)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 666, in run_method
out = Document.hook(fn)(self, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 887, in composer
return composed(self, method, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 870, in runner
add_to_return_value(self, fn(self, *args, **kwargs))
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 660, in
fn = lambda self, args, **kwargs: getattr(self, method)(args, **kwargs)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/gl_entry/gl_entry.py”, line 27, in validate
self.validate_currency()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/gl_entry/gl_entry.py”, line 119, in validate_currency
account_currency = get_account_currency(self.account)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/account/account.py”, line 233, in get_account_currency
return frappe.local_cache(“account_currency”, account, generator)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 1291, in local_cache
local.cache[namespace][key] = generator()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/account/account.py”, line 227, in generator
account_currency, company = frappe.db.get_value(“Account”, account, [“account_currency”, “company”])
TypeError: ‘NoneType’ object is not iterable

Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/app.py”, line 56, in application
response = frappe.handler.handle()
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 21, in handle
data = execute_cmd(cmd)
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 52, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 920, in call
return fn(*args, **newargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/desk/form/save.py”, line 19, in savedocs
doc.submit()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 741, in submit
self._submit()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 730, in _submit
self.save()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 230, in save
return self._save(*args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 280, in _save
self.run_post_save_methods()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 793, in run_post_save_methods
self.run_method(“on_submit”)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 666, in run_method
out = Document.hook(fn)(self, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 887, in composer
return composed(self, method, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 870, in runner
add_to_return_value(self, fn(self, *args, **kwargs))
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 660, in
fn = lambda self, args, **kwargs: getattr(self, method)(args, **kwargs)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py”, line 135, in on_submit
self.make_gl_entries()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/controllers/stock_controller.py”, line 30, in make_gl_entries
make_gl_entries(gl_entries, from_repost=from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 19, in make_gl_entries
save_entries(gl_map, adv_adj, update_outstanding, from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 88, in save_entries
make_entry(entry, adv_adj, update_outstanding, from_repost)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/general_ledger.py”, line 99, in make_entry
gle.insert()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 192, in insert
self.run_before_save_methods()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 772, in run_before_save_methods
self.run_method(“validate”)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 666, in run_method
out = Document.hook(fn)(self, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 887, in composer
return composed(self, method, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 870, in runner
add_to_return_value(self, fn(self, *args, **kwargs))
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 660, in
fn = lambda self, args, **kwargs: getattr(self, method)(args, **kwargs)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/gl_entry/gl_entry.py”, line 27, in validate
self.validate_currency()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/gl_entry/gl_entry.py”, line 119, in validate_currency
account_currency = get_account_currency(self.account)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/account/account.py”, line 233, in get_account_currency
return frappe.local_cache(“account_currency”, account, generator)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 1291, in local_cache
local.cache[namespace][key] = generator()
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/account/account.py”, line 227, in generator
account_currency, company = frappe.db.get_value(“Account”, account, [“account_currency”, “company”])
TypeError: ‘NoneType’ object is not iterable