PHPlist Admin Redirects to “Page Not Found”

I’ve installed WordPress and PHPlist on my server in different sub folders. WordPress is in https://mydomain.com/wp/ and PHPlist is in https://mydomain.com/subscriptions/.

I have no problems accessing the admin pages of PHPlist except that when it comes to “Send A Message” I can edit my mailer, but if I click “send test message” or “save changes” I am directed to my WordPress Template with error “Page Not Found”.

I’m guessing that this might be a .htaccess problem, but I really have no idea. everything is all set, just for this problem.