Stuck Turbo

have you tried mouse milk?

I'm with Robert - try something cheap & simple first.

Not sure what  "mouse milk" is but we have weasels pee (WD 40) or similar. Give it a good soak, leave for several hours (overnight good) then try to move by hand, even a light tap or two with small hammer onto wooden rod/dowel (do not use force).

Advice after the fact is a a bit annoying but be advised that  "2500 rpm in the engine once a month". for any engine (not just aircraft) is not considered to be appropriate strategy/practice, for long storage and is likely to be the cause (or at least contributed too) your "stuck" turbo.

Thanks for your advice and comments. I got it solved with WD40 as you say. 
Luckily my glider is airworthy again and will be flying in a couple of days!