This issue occurs due to the presence of Cisco bug ID CSCsc51939.
This problem occurs when HTTP inspection is enabled in the PIX Firewall configuration.
For a workaround, disable HTTP inspection, as shown:
hostname(config)# policy-map policy name >hostname(config-pmap)#class < class name >hostname(config-pmap-c)# no inspect http
Alternatively, download and upgrade to any of these versions: