It works fine for me. 

Do you have any special characters in your password (i.e., !@#$%^&*()_ ).  If so, maybe they're not getting transferred correctly.  Temporarily changing your password to an alphanumeric would confirm or reject that theory.

Do you enter your username in all lower case, or mixed case?  You may want to play with that too.  I'm fairly sure the forum software isn't case sensitive, but perhaps Tapatalk is, although it would be unusual.  I know I enter my username exactly as it's registered on the forum.

The only other thing that comes to mind would be different language settings (in Settings -> General -> International). 

It is odd that it only happens here though.  That suggests there's something different about either your account here, your account as it's entered in Tapatalk, or something with the way this forum is configured.