"Module not found" error

DEV Env:
ERPNext: v12.x.x-develop () (develop)
Frappe Framework: v12.x.x-develop () (develop)

Production Env:
ERPNext: v11.1.10 (master)
Frappe Framework: v11.1.10 (master)

Currently I’m developing custom app on frappe|erpnext master branches.

Faced an issue when tried to continue development in development environment. Installed the custom app into dev env and result was somewhat mixed. At first time it was ok-ish. I could locate new DocType’s from my app. But had to clean up environment due to other issues. Ran bench update and then installing the app results in ErpNext backend showing error (after logging in):

The console output shows following exception:

Traceback (most recent call last):
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/www/desk.py", line 22, in get_context
19:57:17 web.1            |     boot = frappe.sessions.get()
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/sessions.py", line 133, in get
19:57:17 web.1            |     bootinfo = get_bootinfo()
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/boot.py", line 42, in get_bootinfo
19:57:17 web.1            |     load_desktop_icons(bootinfo)
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/boot.py", line 100, in load_desktop_icons
19:57:17 web.1            |     bootinfo.allowed_modules = get_modules_from_all_apps_for_user()
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/config/__init__.py", line 11, in get_modules_from_all_apps_for_user
19:57:17 web.1            |     all_modules = get_modules_from_all_apps()
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/config/__init__.py", line 21, in get_modules_from_all_apps
19:57:17 web.1            |     modules_list += get_modules_from_app(app)
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/config/__init__.py", line 59, in get_modules_from_app
19:57:17 web.1            |     onboard_present = is_onboard_present(m) if show_onboard(m) else False
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/config/__init__.py", line 78, in is_onboard_present
19:57:17 web.1            |     sections = get_data(module["module_name"], False)
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/desk/moduleview.py", line 33, in get_data
19:57:17 web.1            |     data = build_standard_config(module, doctype_info)
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/desk/moduleview.py", line 120, in build_standard_config
19:57:17 web.1            |     frappe.throw(_("Module Not Found"))
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 353, in throw
19:57:17 web.1            |     msgprint(msg, raise_exception=exc, title=title, indicator='red')
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 339, in msgprint
19:57:17 web.1            |     _raise_exception()
19:57:17 web.1            |   File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 312, in _raise_exception
19:57:17 web.1            |     raise raise_exception(msg)
19:57:17 web.1            | frappe.exceptions.ValidationError: Module Not Found

Is there an easy solution to this issue? (the hard one being creating app in dev env and propagating code from the old one to the new)

Try migrating. What version of python are you running?
Also, are you certain your app is installed? bench version

Development was on 3.5, master on 3.6.

Migrating to 3.6 on dev, but having installation issues. Will report later if issue reproduces in new dev env.