The https configuration is wrong or incomplete. I see wpForo still work with http. Please navigate to Dashboard > Forums > Settings and click on Update options to reset base URL. Then navigate to Dashboard > Forums > Dashboard and click on [Delete all caches] button. Then make sure your site and home URLs are https:// in Dashboard > Settings > General admin page.
Then please don't use https if you've not configured your website for https. It'll affect all plugins. First please do a complete https configuration. It's the wrongest thing to have http:// URL settings for https:// website. Please read docs and configure this from zero, remove all previous configurations and start from scratch. Once your website is correctly configured and your URLs settings are https then you can use wpForo as well.