Hi Matthias,
Please confirm if the listdesigner webapp is correctly updated i.e uim_listdesigner - webapp:20.50 as that the version in 20.4 CU10
https://support.broadcom.com/web/ecx/solutiondetails?aparNo=99112028&os=MULTI-PLATFORM

I dont see log4j-1.2.17.jar in my test env in 20.4 CU10 in the listdesigner webapp

You can try to redeploy ump_listdesigner 20.50 and see if it removes the jar file .
If still present then can take a backup and remove it
Regards,
Frank