Thanks for the hints!
Indeed it might be an issue of the sigil.ini in the user directory.
- Disconnected the networks (8 of them are sometimes connected at the same time)
- Renamed the old sigil.ini and restarted Sigil
Voilą! It opens as I expected.
Compared the old and new sigil.ini and it seemed to be related to some entries in block [mainwindow]:
- recentfiles
- clipboardringhistory
But: after that I took the old sigil.ini again and it opens as quick as the new one, so

Weird.
I'll further investigate this and keep you updated.
*2014-01-02: UPDATE*
It just works. Never saw that behavior again, meanwhile there were way too much system changes, so I don't know the real reason of the issue I had before.
Anyway: Sigil rulez!