Rgistration works normal and with other plugins, but not with Ultimate Member?

Hey, I think I have a very strange issue.
As I read before, there are other users who had a problem with the registration form. I also have had this problem, that my page just reloads when somebody tries to register. I tried registering with another plugin and it worked. The crazy thing about this is: After creating a new user with the other plugin, Ultimate member worked too! When I delete the user I’ve created with the other plugin, the UM Registration form stopped working(??). Also after a few hours, it stops working automatically. Today it worked all night long, but when I tried in the morning, just about half an hour after the last registration, it didn’t work. After creating an account with the other plugin, it worked again.
I never had problems with the registration form before, I am just experiencing this for about 2 days now.

The only thing I recognized is, that the two plugins user different time zones. The other plugin uses my timezone, and UM another one (about 2 hours delayed). So when I create an account with the other plugin, and then one with UM, the one created with UM was created before the other user.

I just don’t understand this logic. I hope that you may can help me. Thanks so much!