You session table seems to be corrupt. This happens, when MySQL has no more empty space on the harddisc (MySQL bug, not ezPublish).
Check your harddisc space. Try to "Repair" the ezsession table (using MySQL command line or some interface like phpMyAdmin). This should work.
If not, you probably must re-create the table (content will be lost, but that's okay). Check (and maybe repair) your other tables, too. If you have one, you could load a backup, but if MySQL runs out of disc-space, the error will re-occur.
