Part of my two-factor authentication add-on requires interrupting the log in process to allow the user to enter their 2nd bit of authentication.
I'm doing this by storing all the input parameters from the XenForo login form in the user's session, displaying/validating the 2-factor login, then restoring the XenForo login form data to the request and passing back to the XenForo login process.
With memcached sessions switched off this all works ok.
With memcached sessions on it no longer works for normal user logins, but appears to still work ok for admin logins ...
After a load of OTT debug messages it looks like the data just doesn't come back out of memcached ...