FC in combination with Knews subscribe widget problem

When using the subscribe widget from the Knews newsletter plugin, this gives me a wrong email address warning. Knews has this option to prevent it: Prevent bot registrations. Some Cache Plugins can need deactivate this option (Subscribe always fails “wrong e-mail adress” message).

But ofcourse I don’t want bots to register and would like to keep this option enabled, so I added the Knews js file to the exclusion rules of FC, but that didn’t work.

Do you have any suggestions what to try next?