There are a few posts out there with people documenting problems with userpics.
I have had a different issue. When upgrading to 4.21 I saved the assets and then copied them over with an ftp client. During the upgrade process these new files and folders were assigned the incorrect owner (should be nobody) which caused plenty of problems for users trying to update their userpics.
So make sure all files and folders in the mt-static/support are assigned to the Movable Type user - in my case nobody - and things will be fine.
Related Tags: movable type movable type 4.21 movable type installation
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