Hi Torsten,
we're fixing this bug. LogOff should really behave like closing/opening the Excel workbook, i.e. you should be prompted for a BI system again.
However: logoff is not like in Bex analyzer where you can assign a new system to the query afterwards - this is handled by the system remapping functionality.
By the way, the long awaited and frequently requested logoff-button has made it into release 2.3 ...
Best,
Alex