I wanted to repost this post from the legacy forum as I found a solution and could not respond there.
https://forum.reallusion.com/553765/Unable-to-add-Actorcore-motion-files-to-CC4-perform-list-editor
Was having the same error “Files with invalid characters cannot be added to the perform list. Only alphanumeric, underscores and spaces are allowed.” After changing the motion file name to adhere it still didn’t work.
Solution: Any other parent folder names cannot contain the invalid characters as well. One of the parent folders had an apostrophe and once changed the motion file uploaded successfully.
Should be a small back-end fix that would help as this was super annoying to debug.