This problem is caused by a bug in SGOS, which is now fixed in the following versions of software:
188.8.131.52 (and higher)
184.108.40.206 (and higher - includes all versions of SGOS 6.4 and 6.5)
Customers running older versions of SGOS software can also use the following work-around, as an alternative to upgrading:
Step 1. Clear all the WCCP settings.
a. First save a copy of your settings
b. Run the command:
ProxySG#inline wccp-settings eof
Step 2. Re-install the complete WCCP configuration, with the desired new mask-value.