This was due to a server space issue. The root drive ran out of space. This is a smaller drive that is used to store saves temporarily during upload, so because of that uploads were failing.

The issue is resolved now, @Simon logged in to fix it.

All saves uploaded while the problem was active were lost. Please reupload them if you want to restore the saves. If the save was previously uploaded, use history:saveid in the search bar to get to the older versions.