Good news. Today I resume the tests. I know that the cache on the main site has been emptied by Siterground and I know that they have moved the server from Milan to Amsterrdam (I would not want a whole series of implications have negatively affected the staging platform, because the site in production had no problem Now do not waste time understanding why, now I resume the development exactly as you left the site and see if everything goes as it should I leave the ticket open and if everything works in a few days, I close it. for the patience.
Please enter admin, Silla left look for the item "-> sleep-hotels" and click on it. Now press "add new". Now he gives me the error again. NB: "sleep-hotel" has just been created. I await info.
The login data has not changed. So I deactivated cloudflare (no result, same error).
Leaving disabled cloudflare I emptied the siterground cache .... it worked again like before.
I hope this helps both you and the users. (moral: when programming, disable the siteground cache, completely)