OK, I did find another problem. This is the first thing in the log that looks like anything is amiss:
mayan.apps.backends.model_mixins <20> [ERROR] "get_backend_class() line 50 ESC[31;1mImportError while importing backend: mayan.apps.sources.source_backends.SourceBackendWebForm; Module "mayan.apps.sources.source_backends" does not define
a "SourceBackendWebForm" attribute/classESC[0m"
This looks similar to issue 1153, but there, the entire package source_backends was missing.
It’s also an error Obelix1981 had in their log in this forum thread, but I can’t tell whether that was what broke the deployment for them.
I also don’t can’t delete any sources, as suggested by ssf later in that thread, possibly as a fix for the above error(?), because the application won’t start.