I managed to fix the issue, so I’ll paste this for future reference.
I had this error message on SAP BTP::
ERROR StatusLogger Log4j2 could not find a logging implementation. Please add log4j-core to the classpath. Using SimpleLogger to log to the console...
When I triggered the error again 12 h later, I got a different message:
Did not expect an argument to be undefined Error: Did not expect an argument to be undefined at l…
Then I googled for that message and I found this forum entry:
Roeland Salij stated that „… this error is most likely caused by incorrect access rules configuration.“
So I checked the access rules and there was indeed the problem. The error popped up for one role but not for the other, something I overlooked during testing. So I fixed the access rules and everything worked.
Room for improvement: Clearly the error message could be more descriptive in the Excel Export module.
And I have no idea why I initially saw only this log4j-core error message.