WordPress won’t save any post


my issue is similar to the one addressed in this question (WP won't save changes in post (any)

Long story short: all of a sudden 3 wordpress install stopped working. I type a post, hit "save as draft" – or publish – and everything seems fine at first look, but actually the post is not saved. If I go back to the dashboard > All posts, the post has been saved with title "Quick draft" and is completely emtpy. Also permalink is not saved. Oddly enough I can save a draft post using the dashboard home feature – the small box that lets you add content quickly from the dashboard home page. I can also upload images but I cannot set a featured image – the loader keeps spinning but the image is not attached to the post.

I suspected a cache issue, so I cleared the cache from my Hosting panel, but had no luck. I tried the Health Check diagnostic, but the plug-in diagonstic was fine. I disabled all the plug-ins, reverted to TwentyTwenty theme, cleared cache again, but still no way to save a post.

I contacted the hosting provider had a chat with a customer care rep who flushed the cache and I manage to save one post, but after that I had the same problem again. WordPress is up-to-date (version 5.5.1) and the dev versions of two of the websites work on my local install – so I don’t think it’s a plug-in conflict. As a workaround I managed to write the post on the local version, export it and then importing on the live website and it works, except that the post doesn’t have a featured image (it doesn’t get imported).

The hosting provider says it’s a development issue and I should ask a developer, since everything’s ok on their side.

Any suggestion before changing the hosting provider?