problems backing up site using snapshot

i am having some trouble running snapshot on one site.
i have used snapshot on another web site on this server and it works great
at first i got a

ERROR: PcLZIP file:media/2015/09 add failed -10: Unable to find End of Central Dir Record signature [code -10]

error so i changed the over the zipArchive method after reading about it in the wpmu forums, and now its stopping much sooner and giving me this error

table_data not set. This normally means that PHP on your server is not properly setup to handle _SESSION. Check with your hosting company.

i have tried going back to the other zip method to no avail.

this forum entry appears to be having the same trouble as i am.

Regards Luke