HTTP::header replace error

You're attempting to rewrite the HTTP Host header using a dynamic method to determine the appropriate string range and then a datagroup look up.

set rewrite_host [class match -value $folder equals rewrite_folders ]

I would first check datagroup 'rewrite_folders' to make sure your datagroup has a corresponding value entry for $folder. This will be in the 'data' portion of the datagroup.

Also check if there are any other iRule issuing a redirect, you may want to consider using priority for irule execution order.