In the forum of Mediamonkey, someone wrote.
Discogs had updated the ciphers across the board to their servers - including the API services - which is what was causing the problems.
They have reverted the changes to the API services, which has restored functionality for the tagger in MM4 on Windows 7 (& Windows 8 presumably)
You can view the threads here... https://www.discogs.com/forum/thread/809378
And here... https://www.discogs.com/forum/thread/809470
I assume, Mp3tag should work again with Discogs too?