Can you export a ref file and use Compressor separately? If Compressor runs separately but not inside FCP trash FCP prefs and retry. If it still doesn’t run then I would be suspicious of the Compressor install.
Also try to run permissions from Disk Utility.
I can use compressor seperately and it works, although it seems very slow, but im not sure if this is abnormal. Can u explain in more detail how to trash FCP prefs and how to run permissions from disk utility?