the url variable restriction plugin can't retrieve the catid value in a content view with the sef mode activated. did you know this ?
Please put a form inside a content view. on one field on this form use the url variable restriction plugin and set it as catid= xx (where xx is the category of the main content)
if you deactivate the SEF settings the restriction plugin can retrieve the catid value. if the sef is on, it can't anymore.