One of the problems was an old laptop with an decrepit BIOS-battery. This led to an totally insane system date at every startup. Even with a proper system time and NTP-Server setup it never set the time correctly as the time offset was to large.
After some searching within the registry I changed under
MaxPosPhaseCorrectiontoFFFFFFFF, which is the maximum value (approx 136 years). Now at every connection, or at least at every manual time adjustment, the time is really set (and no boring error message is shown).