Hello there @KimPossible42!
From your provided log, although you mention this is not a permissions scenario, thefollowing call states otherwise:
Unhandled exception: IOException: Permission denied
Wrapped by UnauthorizedAccessException: Access to the path ‘/opt/unreal-engine/Engine/Intermediate/ProjectFiles/PrimaryProjectName.txt’ is denied.
This means that UE is either trying to access or write on this location, and being prevented by the system to do so. Since your install is located in /opt, the best choice to test here is to move your install to another location, so UE can generate files on startup without clashing with system rights.
Alternatively, you can test the method implemented in the thread below, which encountered and resolved a very similar scenario: