Error / Conflict on BBpress page ?

Hi,

i ve got this on a bbpress page :

Warning: session_start(): open(/tmp/sess_miev7c7vq1tm6spohkc6gfdmduett5al, O_RDWR) failed: Permission denied (13) in /home/.../www/wp-content/plugins/wpmu_dev_maps_plugin/inc/external/wpmu-lib/inc/class-thelib-base.php on line 68

Any idea ?