yes, i already had a previous Snow Leopard, running FCE without problems; after updating it to the last possible version - e.g. 10.6.8 - FCE crashed. i run FCE with an admin account (there is no other account on my MBP). although i have various of your mentioned installations like iMovie 11. iPhoto, GarageBand, it seems to be a problem coming directly with or the update to 10.6.8 or the update to FCE 4.01 (both were coming together, so i'm not sure which update causes exactly the problem, although the FCE update from 4.0 to 4.01 wasn't finished the first time).
however,solution: i made a so called PRAM reset (e.g. pressing Option + Cmd + P + R) during my last reboot (and i was already prepared with all the software for a complete re-install session of everything...), and: it goes!!!
seems that various updates can crash various preferences, including FCE, and a PRAM reset puts all the (wrong) preferences into the right places.
thank you so much for all the help!!