Everyone, thank you for your detailed responses.
Skimming through your provided resources, I’m assuming that files were simply copied, leading to the broken database and that means that even changing the configs and a re-setup of nginx are not going to help.
Originally, the goal was it to have a 1:1 copy of the live server (instead of backup and restore) so we could have the same ERPNext 10 version to have an accurate testing ground for the upgrade to V12.
Our provider, where the server is hosted, said it’s too complicated to just clone the server, so we had to do it ourselves, leading to this mess. I’m probably going to set up an entirely new ERPNext instance, restore a backup of the live system, although our expert warned us that there might be crucial database differences, if we do it that way.
Edit: I could get some more info.
Apparently, it was indeed a full server backup, that we restored in a virtual machine. So from what has been posted so far, this should be the correct approach, just that we should stop the database before copying