I think your forum page is closed for not-registered users. There is no way to open the forum page if you're not logged-in, so there is no way to open the login page as well. Just don't hide the forum page and all will work fine.
Also, this could be a conflict with other plugins, I always recommend deactivate other plugins if something doesn't work. If the issue is fixed, activate them one by one, and you'll find the problem maker.
In case you want to say thank you !) We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.