Please, press F12 in your browser after you open teh administrator, and in the console tab you will see there, check if you see any error attach to there.

I currently have a site with same problem, without able to fix it yet, even after try several hours on it.
So far I blaim the problem to some kind of hard cache in my chrome browser. if I use other browsers that I rarely use, everything works

And then in that console tab a error like this one show up:

Access to XMLHttpRequest at 'http://mysite.com/administrator/index.php?option=com_joomlaupdate&task=update.ajax&0fd45a7182452581ba78a737fa82fc76=1' from origin 'http://www.mysite.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

Even when error say about CORS, I tried all avalaible combinations for setup this, and not fix the problem. But also, others browsers work okay without modify anything, chrome (my default browser) should do too.

@carlitorweb fwiw http://mysite.com is not the same as http://www.mysite.com. The www is a subdomain of mysite.com so restrictions apply...

@dgrammatiko did not see that. But why then open in others browser? Is what confuse me....

With F12 in chrome ,console displays
Access to XMLHttpRequest at
'https://www.nicolesaenen.be/administrator/index.php?option=com_joomlaupdate&task=update.ajax&bd05f19467342b5bf3401f0a70952a42=1'
from origin 'https://nicolesaenen.be' has been blocked by CORS policy:
Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.
www.nicolesaenen.be/administrator/index.php?option=com_joomlaupdate&task=update.ajax&bd05f19467342b5bf3401f0a70952a42=1:1
Failed to load resource: net::ERR_FAILED

Check https://observatory.mozilla.org/analyze/nicolesaenen.be

Fix this (it's the server setup) and you'll fix your cors problems

@jobrusche Please check @dgrammatiko 's comment above and report back if it helped you to fix your CORS problem. Thanks in advance.

Added the following to .htaccess in the section IfModule mod_rewrite.c

RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}$1 [R,QSA]

All symptoms are solved
Thanks alot.
Johan

Closing as not an issue with the CMS core. Thanks @jobrusche for reporting back.