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'.