Seahub not working in fastcgi mode

i used to have a stable and working setup of seafile 6 with apache as the frontend server communicating to seahub over fastcgi. Since some days (not exactly sure when) my seafile server was unreachable.
The seafile server in general starts up nicely, however seahub is not accepting connections on port 8000 internally any longer and mod proxy disables the worker for the fastcgi connection as seahub on port 8000 refuses the requests.
There is no indication in error log or the other logs of seafile.

I found out that seahub is working without problems when I start it in non-fast-cgi mode- is is then accepting connections internally on port 8000 (have not setup the mod_proxy properly for external access though).
One related topic might be problems in resolving the hostname for seahub but this is now fixed and seahub is still not accepting connections in fastcgi mode.

No further change in the infrastructure has been madeā€¦

Any hints how to debug this issue further?

UPDATE: I have now also changed the apache2 / mod proxy setup and it works again- but not over fast_cgi