Comment 1 for bug 1446036

As a workaround, reverting the changes to htdocs/auth/session.php corrects the problem:

git checkout 55a8deb8cb~ -- htdocs/auth/session.php

This will cause the ajax status bar to not load properly, and it will cause the ajax block loader to load blocks in serial instead of in parallel, but otherwise it causes no problems. You can disable the ajax block loader by adding "$cfg->ajaxifyblocks = false;" to your config.php file, which will turn off the ajax block loader and prevent that problem.

So far I have only noticed this problem popping up while doing an mnet connection, and only on our proxied hosting cluster (not on my local machine). It's possible that MNet sets a lot of session values, thus causing the problem to be worse than normal. It's also possible that our hosting cluster has tighter response header limitations than my local machine. However, the potential exists for this to cause problems in other areas and other hosting setups as well, so I've marked it "Critical".