ISA 2004 console > Configuration > Add-Ins > Web Filters tab > properties
of compression Filter > select Enable this filer. > Ok > Apply.

try way 2:
ISA2004 error 12217,Help me!!
取消验证正则化
如果URL有高位字符请求,同样要取消选框
把HTTP过滤器禁用了试试

详情请见:
http://www.microsoft.com/china/technet/prodtechnol/isa/2004/plan/httpfiltering.mspx

Error Code: 500 Internal Server Error. The request was rejected by the HTTP Security filter. Contact your ISA Server administrator. (12217)
1. 打开ISA Server控制台.
2. 展开ISA Server.
3. 点击防火墙策略,选择需要管理的策略.
4. 点击右键, 点击配置HTTP.
5. 取消阻止高位字符的选项,点击确定.
6. 点击应用更新策略,点击确定.