Within specific forums, when we change from Condensed View to Expanded View, the page returns a Http Error 500. We can still access the second page of that forum by manually entering /page/2/ to the URL, and from there changing back to Condensed View to be able to get back access to the first page.
This is happening to all our users, across all devices and themes. Those specific forums can be viewed if the user logs out though, since the forums are in Condensed View by default to guests. If you log back in while within those problematic forums, it takes you to the second page.