It looks like you're getting the 200 response after following the 302 redirect (means login request was successful). When this happens the cookies are set to $null (because none are received in the subsequent 200 response), so that explains the behavior you're seeing. But it shouldn't be following the 302 to the 200 with the latest jsonformirc, can you enter the following command? Modifying the path if you don't have these in the mirc script directory.
//echo -ag $jsonversion :: $md5($mircdirscripts\Json For mIRC.js,2)
SReject/JSONForMirc v3.0.1002 :: 230784d5928a75c2fb92a6e41f983b9f
Included is the current version and hash at the time of this comment.