In wikibase.cloud edit function is not working on wiki pages: 'visual edit' is working, but 'edit source' is not.

When saving an 'edit source' operation you get always this error message:

Sorry! We could not process your edit due to a loss of session data. You might have been logged out. Please verify that you're still logged in and try again. If it still does not work, try logging out and logging back in, and check that your browser allows cookies from this site

If you use for the same page the 'edit' function it works and you don't get the previous message.

Please note that since there are pages like https://[wikibase.cloud]/wiki/MediaWiki:Wikibase-SortedProperties where you can use only the function 'edit source', it is impossible now to modify these pages

Steps to replicate the issue (include links if applicable):

• try to edit - using 'edit source' - a wiki page of one of the instances of wikibase.cloud:
• press 'save changes'

What happens?:

You get always this error message:

Sorry! We could not process your edit due to a loss of session data. You might have been logged out. Please verify that you're still logged in and try again. If it still does not work, try logging out and logging back in, and check that your browser allows cookies from this site

What should have happened instead?:

Wiki page saved successfully

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia): 1.39.7

Other information (browser name/version, screenshots, etc.): I replicated this bug using current versions of Chrome and Edge

Probably linked to: https://phabricator.wikimedia.org/T376853