I get the same error with FO4 a lot. I don't use MO anymore though. Only the Nexus Mod Manager. Also, I never use the XEdit program through NMM or when I used the MO, didn't use it then either. A lot of times I got it to work was by rebooting ( NOT a logout ), turning off any background Virus scanners and defrag programs and cleaners... just to clear out ANY used memory. Sometimes it just didn't work. The MOD would work fine but XEdit just wouldn't do anything but give errors. Meh. Just a suggestion.