Hi Ashish,
Thanks for your reply.
Anyway, the problem has been solved / disappeared and importing a Fitness Center solution (e.g. FitnessCenterCommon_1_0_0_0_managed) passed OK. It seems that applying the roll-up update 6 for CRM 2011 has solved the problem. Last day it occured that Windows
update has suggested to install Rollup 6 update for CRM 2011. After successfully installing that update - as said - the problem has been solved / disappeared.