Hopefully, this will save you some time. I reported a ticket and closed it, after realizing the actual cause. Domain DNS managed in Cloudflare.

====ticket=====
After the upgrade, it is impossible to edit a page using the actual domain. The picture is attached and very likely it is due to mixed content:
Mixed Content: The page at 'https://www.kickbox-bg.com/web#menu_id=113&cids=1&action=396' was loaded over HTTPS, but requested an insecure frame 'http://www.kickbox-bg.com/'. This request has been blocked; the content must be served over HTTPS.

There is a workaround - deleting the domain from setup and editing content properly, then bringing it back.

This is observed in several databases.

====ticket=====
Solution is to make sure below settings are:

Cloudflare --> Domain --> SSL/TLS--> Overview--> 
1. Your SSL/TLS encryption mode is Full

2. SSL/TLS Recommender - turn it ON