You do not need to deactivate the cache plugin but you DO need to exclude the forum page form ALL of it. There is an exclusion section for each cache type where you should put the forum slug.

This still sounds lie a javascript AJAX issue to me. It is reloading the entire page instead of retrieving the data with an AJAX call. So we still need to see what is showing up in the console  script error. Although if it another WordPress plugin we still might not be able to identify it that easily.

Have you considered the usual process of briefly switching to the WP default theme and then – if that doesn’t change anything – deactivating WP plugins one at a time?