When adding new pages the + (plus) symbol in the navigation tree on the left disappear. I'm only left with "expand/collapse all" functionalities.

Also today on my first visit to the site (which is the portal 0 default site), where I added several pages, all the pages where 404 errors (including the pages that were already there before the upgrade and were not added). Luckily I had another site in the same DNN install, went there, went to host section changed the portal 0 "not-working-anymore" site to another alias, then revert it back to it's original alias and automagically the pages started all working again.

But there still be some admin/pages/urls related issue in 7.3.1. Do the plus sign disappear from your installs when you add pages at least? This could be a starting point to troubleshot the issue.

It happened again, this time on portal 1 (the 404 error for all pages except home). I tried restarting IIS and it worked. Just wanted to update you that IIS trick worked so this should narrow the bug and help find a solution.

Do you see anything in your DNN Event Logs and the Log files that Greg pointed out?

Also check your servers application logs, you may not find anything but it's worth looking.

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