Thank you for the compliment about my site. Come to think of it, I had a similar problem. In fact if my memory is correct, this happened from A4. The problem came from a third-party module => https://una.io/view-product/force-avatar-upload.

I had to deactivate this module and restart the complete installation. Here is a track, I do not know if you have such a module. Otherwise, it also sometimes happens that the token expires during the update, it happened to me. In this case, only a part of the updates happens. Make sure you still have access to the "Storefront", if necessary reconnect. And double check if there are not some module updates left.

Then, go to the home page (config) and disable one by one the blocks present, to try to determine the block that is causing the problem.

If you have ssh access. Watch the live log with the commands:

echo > /var/log/apache2/error.log (to empty the log file)

tail -f /var/log/apache2/error.log ( for real-time errors )

Refresh the problem page and see what error it produces.

But above all, be sure that there are no updates left.