Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
DearJuho ,
Thank you for posting this question in Microsoft Community.
Apologies for the inconvenience caused and please don’t worry, let’s work together and try to move in the resolving direction.
As your mentioned that the files are stored on shared drive that is on a server, are the shared drive was network mapping from SharePoint or not?
The file stored on shared drive, it isn't support co-edit in Office app. if one user open the file, other user will open it will be read only and make a copy to edit.
For your situation, suggest you store the file in OneDrive or SharePoint library, that support co-edit in Office app.
If you need any further assistance or any misunderstanding from our end, please feel free to post back to us.
Thanks for your patience and understanding. Have a good day!!
Best Regards,