@ all in this thread ...
This is a very strange issue. Can't imagine what one could do to get another server to do the same ... ie, replicate the problem ... which is what one would need to do to file anything in tracker.
So, here's a suggestion ...
1. With whatever tool you have to browse files (that also has the ability to remove files), empty your moodledata's cache directory ... just the files contained therein ... leaving the cache folder/directory. Do the same for 'localcache' and for 'sessions'. The cache/localcache and sessions will rebuild as site is used.
2. login ... go ahead and download the backup *IF* presented with that prompt again.
3.. in 3.1 and I think in 3.2 there is a setting for the 'Default Home Page' for Users' ... that's all users ... including those that are stie admins. Site Admin -> Appearance -> Navigation. By default that is set to 'dashboard'. Change that to "Site". Log off then login again. IF you can't navigate menus, then using browser URL line: http://yoursite/admin/settings.php?section=navigation
This is really a guess ... no known way to replicate the issue but obviously with y'all reporting the same behavior, there is an issue ... just not wide spread.
'spirt of sharing', Ken