Hi Lekart,
Best I can gather at the moment is that it is possible a template issue not this plugin directly. However I was unable to locate the call correctly that was causing the issue. It is only a warning although it stops everything form working.
My solution was to edit the wp-config.php file and add this at the end
@ini_set(‘display_errors’, 0);
This suppress’s the warnings and should be used on most sites anyway once they go live. Solved my problem in the short term… Hope that gets you out of trouble for the moment.
Ollie