Hi megas
I'm sorry, but I'm still a little confused about what u installed when and how.
The only way I can imagine, this error message can be caused by jOpenSim is when you lost only partial group data while reinstalling. jOpenSim stores data for groups in 7 (!) database tables and they should reference well together. If data of one table is lost, most others are pointing to "nowhere" then.
Specially the table #__opensim_groupactive is called during login, responsible to give the user the last active group again. If there is an invalid/broken reference, I can imagine such a message during login.
If your group data got lost when reinstalling, I think the best solution is to empty this table as well. Users with an existing and working group will not loose their membership, but might have to reactivate their current group manually when next time logging in.
Greetz
FoTo50