Error with Quicktab Format after upgrading from Views 7.x-3.0-rc3 to Views 7.x-3.0 : Illegal offset type in views_plugin_style->

I had opened this error under the Views Module (http://drupal.org/node/1378966).
The maintainer posted

It is entirely possible that the update uncovered a bug in quicktabs Views support that was previously unknown.

There is a reason we have procedures like this; I am unable to debug a problem with quicktabs. You state that the error does not happen with the table style, and that is a great indicator that this has to go through quicktabs first.

Ok lets describe the error:
Instead of the "quicktabed" output I get many times (also in the log)
Warning: Illegal offset type in views_plugin_style->render_grouping() (Zeile 312 von /home/www/cleantech/sites/all/modules/views/plugins/views_plugin_style.inc).

Working Environment before update:
Drupal Version 7.10
ctools 7.x-1.0-rc1
Views 7.x-3.0- rc3
Quicktabs Version 7.x-3.2

Change that generated the error (it was the only change):
upgrading from Views 7.x-3.0-rc3 to Views 7.x-3.0

Some little debugging that might help
This View worked under Views 7.x-3.0-rc3 and the error appears directly after updating Views
( and course after running update.php).

I found out that the error occurs when a Grouping Field is defined in the Styling Options,
deactivating it cancels the error but of course the Output is not useful in this case.

The error also disappears when I use a "Table" even with the grouping Field selected.

Summary:
The error occurs with the combination of
Quicktab Format & a selected Grouping field & Views 7-3.0

Link to the table Version of this View http://www.cleantech-seiten.de/zertifikate
where in the quicktab version the tables are in 2 separate tabs.