Watch folder not working after update to version 4.6

After upgrading to version 4.6, I noticed, that the defined watchfolders do not work anymore.
The type entries are all “NULL” for watchfolder sources.

On startup, I get the following error from the system:

mayan.apps.backends.model_mixins <26> [ERROR] “get_backend_class() line 50 ImportError while importing backend: mayan.apps.sources.source_backends.SourceBackendWatchFolder; Module “mayan.apps.sources.source_backends” does not define a “SourceBackendWatchFolder” attribute/class”

I suspect this error is connected to my problem.
How to solve this?
System is docker on debian linux bookworm

I am not sure if it aligns with the uplift to 4.6, but I had to recreate all of my sources around about that time.