The "ICAP/1.0 500 Server error" message is generated by the ProxyAV when the license expires. This message is reported to the ProxySG when it perfoms a health check on the ProxyAV.
The following PCAP taken on the ProxySG shows the "ICAP/1.0 500 Server error" response from the ProxyAV to the ProxySG when it perfoms a health check.
Following an example from a ProxyAV running an expired Kaspersky license:
X-Error-Details: Antivirus license expired; File: No file name available; Sub File: No file name available; Vendor: Kaspersky Labs; Engine error code: 0x00000000; Engine version: 184.108.40.206; Pattern version: 111023.220900.11042785; Pattern date: 2011.10.23 22:09:00