Hi,
Quite sure it is due to memory:-
See the memory requirement section of the release notes:
http://www.cisco.com/en/US/docs/security/vpn_client/anyconnect/anyconnect31/release/notes/anyconnect31rn.html#wp44036
"The minimum flash memory recommended for all ASA 5500 models using AnyConnect 3.1 is 512MB"
Thanks and Regards,
Vibhor Amrodia