Thanks a lot for the quick replies!
@: Sorry, the old esxserver is a standalone Server, not in a cluster like the new one. The new 5.1 Cluster consists of two esxserver is in the same network but not i cant manage it with vCenter Server and so on. Thats why i cant use the migration function.
@: You mean, that an old task could be the reason why the new task doesnt work? I'll have a try, so i will uninstall the converter and reinstall it.