First off let us just double-check the forum permalink is correct. Go to the forum admin > integration > page and permalink. Click on the update permalink button in the middle of the form. Then try a page load again.
If that does not change anything – and if you can post then it almost certainly will not – there are a couple of things we have encountered before that can do this.
The first is a redirection plugin – although to be fair it is quite a while since this one came up. But… there used to be a popular plugin named simply, if I recall – ‘permalink redirect’ which needed the forum page to be added as an exception. Do you have anything active like this?
The other possibility is a security plugin. Some of those can be quite intolerant of extended WP permalinks. Anything like that?
One I would like to add – although again if you can load the pages then I am doubtful but worth checking – would be if the host has turned on the ‘mod_security’ server module. It would be worth checking with them and asking, if it is on , if they can turn it off to run some tests. If that proved to be the issue then they can whitelist the forum URL.
[Update] Also check your .htaccess file. I know that an incorrect entry, sometimes put there with the best of intentions, can cause this problem. Afraid I am not able to advise you on the actual content but again – your host support should be able to.
