Cannot open map in Map Viewer but works in Map Viewer Classic

[TimSexton1]

I just upgraded my portal from 11.0 to 11.2.  Everything seems to be working for the most part but when I attempt to open an existing map in my portal's content, it will only open in the Map Viewer Classic and will not open in the "Map Viewer".  I also noticed that I can no longer "Add New" in the portals content section, just a blank window opens.  I'm wondering if these are related?
New item screen is blank

[TimSexton1]

Disregard this issue. I'm leaving it here in case others experience a similar situation.  Evidently after upgrading everything you need to clear your browsers cache.  This fooled me because it was working after upgrading the server and portal, then I upgraded my web adaptor (named it the same) and after that is when the trouble started.  Web apps 101 - clear the cache!

[ODWC_GIS]

@TimSexton1  I've cleared my cache twice now, and I still encounter the "There was an error" whenever I try to open Map Viewer in my Portal 11.1 site.  Map Viewer Classic still works.  Any suggestions for when clearing the cache doesn't resolve the issue?

[TimSexton1]

In 11.2 I'm actually still seeing some issues with the "Map Viewer" where the "Map Viewer Classic" works and the new one does not.  Have you tried an incognito window to 100% eliminate caches all together? Some times clearing the cache in the standard browser alone doesn't seem to clear it all the time.

[TimSexton1 · Accepted solution]

I had to set a web context URL in portaladmin that pointed to my custom dns entry for portal. This fixed the Map Viewer issue.  I learned this at DevSummit.

https://enterprise.arcgis.com/en/portal/latest/administer/windows/using-a-reverse-proxy-server-with-...