I also have this problem with group windows disappearing. For me, the default group window works fine, but I don't like the horizontal format. I am trying to use the xml files found in _eq2ui_mainhud_groupmembers (vertical).zip for example.
Initially I had no group window and wasn't able to locate it by cycling F10 or Alt-G. I was finally able to make the group window appear by editing _ProfitUI_Settings_GroupWindow.txt and set Background Opacity to 100
However, the window is always empty and does not show my group members or any clickable elements. Just empty black box...
This happens with the following "alternate" UI files:
_eq2ui_mainhud_groupmembers (horizontal compact)
_eq2ui_mainhud_groupmembers (horizontal)
_eq2ui_mainhud_groupmembers (vertical compact)
_eq2ui_mainhud_groupmembers (vertical)
The (default) and (compact) versions work OK but again, I don't like the horizontal format.
Any help would be much appreciated