I am having this same problem. I have searched for an answer and didn't see one. Did I miss the resolution some where? If I did, please xcuse me. I'd appreciate some direction.
I get this same error for Yahoo tagger. Although I don't use very often nor do I like this tagger, I just mention it because it has the same problem. Discogs is working fine.
I uninstalled MM & reinstalled. I tried restoring from a backup using backup v5.8 and all 3 scripts worked fine. Because I have never restored before MM displayed all of my files like 4-5 times so I again uninstalled & reinstalled MM and installed all 3 scripts first. I again am getting the same error. I copied the Musicbrainz folder from the backup to the Musicbrainz working folder and now the script does the search okay but the above error screen still appears. The progress bar also looks corrupt.
The problem is this script was created for MM3, was never updated for MM4.
You can make it work in MM4 but it requires to be installed a special way. First, you must run MediaMonkey 4 as the administrator (right click MM icon > Run as administrator). Then install the script, but uncheck the option to "Install for the current user only".
After that restart MM as you normally would, should work at that point.