Seems to be a duplicate of https://forums.bitfire.at/post/9654, please follow up there.
Currently it seems to help to uninstall DAVdroid and install it again.
When I add an event on my phone for today (25.10.) 19:00 o clock it gets shown as 18:00 on the posteo website. If I try the same with thunderbird, everything works normally. I dont usually have this problem, my old events were correct, so I guess this problem has to do with the fact, that tonight the clock is being reset.
Hope this helps!
Thanks for reporting the issue. This is probably a duplicate of #186, please follow up there.