If you see this error in your c-icap server logfile, it might just be that c-icap is running out of temporary disk space and that the clamav/virus scanner configuration for c-icap is wrong:
Service antivirus_module virus_scan.so ServiceAlias avscan virus_scan?allow204=on&sizelimit=off&mode=simple virus_scan.MaxObjectSize 5M TmpDir /tmp
The option "... sizelimit=off..." for the virus_scan service means that the configuration value for "MaxObjectSize" will be ingored. If you have too many parallel squid client connections open or large files to download, c-icap is running out of temporary disk space. It will then log the following error message without further explanation:
Bug in the service. Please report to the service author!!!!
The webbrowser download will be terminated with an error message (something like "internal server error").
To solve this problem, add more free space to the partition where TmpDIr resides, and change the virus_scan service option to "... sizelimit=on ...".
In the worst case, free disk space for the c-icap TmpDIr has to be:
MaxServers * ThreadsPerChild * virus_scan.MaxObjectSize