I removed the tag group from the ~siteuser/etc/check_mk/multisite.d/wato/hosttags.mk, regenerated the configuration and restarted the core. I got the built-in tag group agent and anything fine with monitoring. All hosts still have the correct tags.
Unfortunately the update process still trows the same stack trace.
EDIT:
I found two hosts haven’t set any of the agent tags, fix this and will try the update and cleanup again. This is maybe causing the problem.