@robert The website in question is for a club and can only be accessed with a username and password. I can send you a username and password, but if possible, I’d prefer to do so via private message if that’s okay with you. That way, you’ll be able to see how it works.
I'd also recommend update to wpForo 3.0.9 version and check it on a new browser. Please do that and let me know the result.
I update to 3.0.9 and test with new browser but still the same 😥
Ok, it's clear. Some of your plugin generates error to the header of page loading request and crashes cookies. This could also be security plugin hard configuration issue. It means all plugin that use cookies will fail on your website. This is not wpForo specific issue this is the website problem that you should do deep audit by disabling all plugins and testing one by one.
I have disable all plugin exept wpforo and it is still the same.