This plugin is causing 2 critical errors

I had to mirror my website and deactivate/reactivate every plugin one by one, to finally narrow it down. The Favorites plugin was causing 2 critical errors in WordPress:

An active PHP session was detected: A PHP session was created by a session_start() function call. This interferes with REST API and loopback requests. The session should be closed by session_write_close() before making any HTTP requests.

The REST API encountered an error: REST API Endpoint: […]/wp-json/wp/v2/types/post?context=edit
REST API Response: (http_request_failed) cURL error 28: Operation timed out after 10001 milliseconds with 0 bytes received

This was causing extremely strange symptoms that looked a lot like a cache problem, but wasn’t: edits/changes weren’t getting published correctly, or overwritten, users sometimes had to refresh to see their edits, etc.

I am absolutely positive this was being caused by the Favorites plugin by the way, as I’ve tested it multiple times: if I deactivate Favorites, the 2 critical errors disappear, and the symptoms disappear as well. If I reactivate it, the critical errors come back, as well as the symptoms.

Theme used The7 version 11.4.0, WordPress version 6.1.1.