I did wait for another week to make sure that I did not mess something up and now I can confirm it wasn’t me:
I set up a reminder to weekly on every Sunday at 5 am and it started last Sunday. Surprisingly it became active yesterday (Saturday) afternoon (zipping a backup of my main database). Then I set it manually to the this morning. It worked but the next due time displayed is again next Saturday.
So I suppose somewhere is a +1 missing.