thanks for the quick answer!
It looks good now. Our support partner finally had the idea, though the reason was actually simple and our own IT was pretty close - we don't have an actual QlikView_administrator...
The reason was simply this: On the server, we have set up a specific QlikView_user that has all the access_rights necessary. This user, however, was created like any regular Windows-user and thus its password expired after a certain period. I changed it yesterday - it seems to have expired on Sunday - but apparently, it can/ must be specified in the management_console also so that the user can actually start the QlikView_services.
We are awaiting tomorrow's automated runs, but we are now "cautiously hopeful", as the politicians say ;-)
Thanks once more!