I have multiple categories, some of them are "Erledigungen","Sport+Freunde". they are displayed in the order I typed them here.

Let's say i create an event in the category "Sport+Freunde" and save it.
Then i reopen the event.
Now, in the event's details, the wrong category "Erledigungen" (the one above "Sport+Freunde" in the dropdown menu)  is preselected. 
Unless I manually correct the category to it's original value, xfce now saves the wrong category when closing+saving the event.

I didn't notice this bug in 4.7.5., however I also created new categories directly after the update.