Something went wrong when changing admin name

I was logged in as Administrator and wanted to change the name of the account.

I changed it, but the system kept the old name for some reason.

But it's not the big problem. The big problem is, that now I'm not able to log in to make system changes anymore.

The name and password is wrong now. I tried the old and the new name.

For some reason I stil can log in when the system is starting...

What can I do now to fix the problem?

Thanks for your response.