Hi...
I just tried a variety of different motion files (.rlmotion, .imotion, fbx) using the Perform List Editor and all were accepted without issue (see below for an example).
Are you using the latest version of CC4 which is 4.42? If not, please update and try again.
And just to eliminate an issue with the motion files themselves, perhaps you can send one of the problematic ones to me via a PM and I will test it.
You might also want to consider uninstalling CC4 and performing a clean install of the latest version in case of an issue with your current installation.
Peter
Forum Administrator
www.reallusion.com