Yeah, the issue is 100% related to the cookie file. I suggest you to repeat the procedure, unlog from r18.com, re-do the login, save the R18 cookie file and then put it in the jav-it folder (replacing the old one).
You can re-download the zip file with the program and the readme checking your e-mail and looking for the mail titled "Your purchase of Jav-it" from itch.it, there is a link to your personal download page there. If you can't find it (not even in the trash bin or in the spam folder) you can re-request it at https://itch.io/support