During the last 5 days I did some tests with old save files from my old game. I tried different combinations of events (even 3 dates the same day) and the result was always the same: The day the save file started with, I could arrange several events (mostly dates) without any problems, but from the next day on the issue with dates and weddings/marring himself appeared. On the other hand, after starting a new game, that issue didn’t appear even after three days of gameplay with a lot of dates, other events and a wedding. And it didn’t matter whether I was playing with all my usual mods or none.
Although this is a strong indicator, that the issue might be caused by the changes introduced with patch 0.6, I didn’t try to play with the patch 0.5 as suggested by @HeinziDerHeld.
It’s a pity for legacy style players, but such profound changes often happen during early excess.