Ah OK. That is still probably the result of a 404. So basically what you seem to be saying is that forum URLs are being processed by WordPress as if they were blog post URLs. This can be down to a few things.
The most probable is that the WP rewrite rules have become scrambled for some reason. Go to the WP Admin > Settings > Permalinks and update the permalink panel. You don’t need to make any data changes. After this go to the forum admin > Integration > Page and Permalink panel. Make sure the correct page is selected at the top. If it is not then select and update. After this click on update forum permalinks.
If none of that changes anything then do you have any redirection plugins? If so then you may need to exclude the forum page (often using the slug). Another culprit can also be the Yoast SEO plugin. Do you have that active?
Next – do you have any cache plugins active as if so you will need to exclude the forum page from being cached. caching should not have this effect but it needs checking.