Page not loading only when logged in as admin

Hi there, I just updated some plug-ins and now have a strange problem: After I logged in as admin, previewing a page or the site won’t load, just showing a loading circle. However I can edit a page or post without any problem. And the changes are confirmed to be made in a different browser visiting the site as a visitor.

It happens to both Firefox and MS Edge, so it’s not a browser issue. I also have tried to clear all cache to no avail.

I’m sure other people have had the same problem before, but I just couldn’t find anything here or by googling. Can someone please give me a pointer? I think it might be related to session control (loading the admin header).

Thanks!
John