Our previously smooth running membership sign-up process failed leaving me scratching my head after a WP upgrade to 5.5. I had made some changes to the form at the same time and was blaming myself for a silly misconfiguration rather than blaming the upgrade. Reverting to a back up that still had wp at 5.4 has solved the issue for now. Anybody seen similar, got a solution?
Got this on the WP site health dashboard
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.