Check the timezone set in PHP, the operating system and memcached
Timezone differences can cause sessions to prematurely timeout due to differences in the time considered to be now.
Disable browserfinger print settings
Occasionally, when a user is accessing the site on a mobile device their IP address can change frequently. If
REMOTE_ADDR is included in the browser fingerprint configuration this will lead to their session being prematurely ended.
Session ending before xforms_session_timeout or session_expiry_time length
If the session is ending prior to the session length specified in configuration, check that
session.gc_maxlifetime has been set to a value greater than the application session length.