Instability after WordPress upgrade, different per admin user

We have a WordPress-driven website with a separate staging site to test upgrades and new plugins before going live with them.

We recently upgraded the staging site to 3.5.2, and installed all plugin updates as well, to make sure everything running at the latest version. Since the upgrade, admin users in the back-end have been seeing:

– ‘Dashboard’ shows only partially up to Recent Comments (some users see the Dashboard title and left nav with nothing else)
– ‘Posts > All Posts’ shows no posts in list (some users see 1 or 2 posts of the almost 1000 in the db)
– ‘Posts > Add New’ shows “500 Internal server error” (only some admin users)
– ‘Plugins’ shows blank white page (some users see all plugins)
…

The admin users who seem to have the most problems are the ones who login to the live site daily and manage posts regularly.

Now, we’ve been troubleshooting the staging site’s issues since the upgrade. So far we’ve tried dozens of different setups, even after debugging the code line-by-line, to try and find the issues with no luck. We have disabled all plugins, increased the WP_MEMORY_LIMIT, increased the session wait_timeout, checked functions.php (and other files) for empty white space at the end of each file, and many other suggestions mentioned on other forums… all with no success.

I was hoping someone else had this same problem and might be able to give some insight into what’s going on. I suspect it has very much to do with the admin user accounts. We’ve been able to solve another similar problem previously by re-creating the admin accounts from scratch but it is a major undertaking which I would not like to go through if possible to re-connect all posts to the new accounts.

Any help would be greatly appreciated! Thanks!