Hi Jens,
The time for the “Created:” field in the UpdraftCentral tab is in UTC, whereas backups are shown according to the WordPress timezone. Assuming that your timezone is UTC+1, those two times are the same, so it sounds like the problem is somewhere else.
Looking at the code, as far as I can see, only the server time matters (so, even if wrong, if it’s the same server, it should be consistent), and it allows a 3-hour window (so even if there was a timezone error of 1 hour, that should not be involved). So, I am puzzled. It may be that timezone is not involved at all – the place in the code where that is checks other things too. Can you try again, and copy/paste the key into this thread (as a private reply), and confirm that it failed when attempted?
David