Its a sortorf bug, but its self made..
The form is in a phrase and the language file you are using is a language file from 2.x.
the file works, except for this phrase as it contains the password form.
But in vbgallery 3 the form has changed, securitytoken was added.
In your old translation securitytoken is missing, => you get a blank page.
Loggin in works in english, but not in german.
to fix, retranslate the phrase: adv_gallery_categorypassword
Respect the html from the english version, and you will be able to login.