Problems with registration process


First of all, I want to excuse for my bad English. English is not my mother tongue but I hope you understand me.

I use the Membership-Plugin 2.0.3 with the newest version of WordPress. I'm quite sure that I configured everything well. But the registration process doesn't work.

I log out of my admin account and try to register as a new member on this registration page:

I fill all required fields (username, email address, etc.) and click on "REGISTER MY ACCOUNT". But I don't get to the next page - the registration page simply reloads. I've tried out a lot of things in the last couple of hours - and still I cannot find the mistake.

Any suggestions? I appreciate any help! Thanks in advance.