If you are getting the “Session invalid or expired” error whenever try to login to your webmail, You can fix it by following simple steps given below.
Login to your server through SSH
Edit the following file:
Comment the following line from the file:
$rcmail_config['session_lifetime'] = 10;
Also you may try to increase the default value: 10 to a higher value.
Save the file and try login to Roundcube again.