Hey Waylander,
I found the same problem with my new GT5, the clutch wouldn't disengage.
 What worked in my case-
 -pull clutch case cover
 -remove flower, or star nut stay screw
 -remove flower, or star nut
 -pull clutch plate, with spring
 -lock rear wheel in place (a screw driver works)
 -remove nut securing clutch drum
 -at this point I tried to pull clutch drum off with my fingers, and, click, the clutch freed up.
Through my reading on this site, apparently storage 'helped it get sticky'.