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[sinti]

Now, WHAT didn't work? Please describe the behaviour!

PS: No need to quote my whole message then. Did you know, the Preview Button next to the Post Button is very useful.

[BenTexas]

Thank You very much Sinti! 
This fix works fine for my installation.
I had to change a few things though, because I use a bridged Version with PHPBB:
I have different groups enabled and the USER_GROUP constant from Sinti's Code was only containing "Registered" no matter what other group a user was in. So I took a closer look at the USER_GROUP_SET constant and found out that the different groups from PHPBB are stored inside coded with a numerical value. To find out the mapping, I created a test user, found out what number belongs to what group and then tweaked the Code to check for a certain group number. Works fine for me!