iRules "Operation not supported" log errors

From the HTTP::header insert help page:

"If this command is executed after issuing the HTTP::redirect or HTTP::respond command, the F5 will become confused, generate an "Operation Not Supported" TCL error and reset the connection."

Do you have another iRule assigned to the virtual server? If so, take care with the order and possibly use priority to ensure that this runs first.

You should also check whether the header is already present using HTTP::header exists and then use replace to replace it. Basically, add some error prevention code around the insert.