Not 100% sure on that one.
But it does work... On my test forum with PHPBB conversion.
Produced a 'Test' group... then through a persons profile added them fine.
I rcv both the php links that you do... after the last one it goes back to page straight away and is updateded.
*** Maybe someone else can help you with this error... over my head.... ***
BUT... if you need an alternative.... (This wont benefit if you gain users reguarly and change groups for people often)
You can change it in your mysql database by changing the corresponding user on the 'Group ID' field in the 'Users' group.
To find out the corresponding number for your new group. Check out the 'Group ID' group.
Standard is 1=Admin, 2=Mod, 3=Guest, 4=Members
Any new ones created will start from 5... <-- This is probably the one you have just created.