In PHPCMS V9.1.8, frontend member registration fails with an “Operation Failed” prompt. Clearing the cache in the backend does not fix it, and the automatic upgrade in the backend also fails (possibly due to server configuration issues).
I then performed a manual upgrade all the way to the latest version, but the problem persisted. After a final inspection, I found the issue was with phpsso_server. The database password may have been changed at some point, but the password in phpsso_server was not updated accordingly, causing the registration to fail. Check the file: phpsso_server/caches/database.php