It seems like there are some issues with the user and group permissions that Froxlor is trying to set up. A few things to check:
1. Make sure the 10000 and 2000 uid's that are being assigned actually exist on the system. Those uid's may have been valid on a previous Ubuntu release but not 20.04. You may need to adjust them to actual uid's that exist.
2. Double check that the file permissions Froxlor is setting (like drwxr-x)