In the sitewide forums(bbpress) configuration there is an item that is checked by default to prefix /forum/ to permalinks. This changed my permalink to /forum/forums/forum_name which broke the links. I unchecked this which made my permalink the normal /forums/forum-name and fixed the blank page issue for me.
I had this same issue with buddypress+sitewide forums. I resolved by unchecking the box in forum configuration that added /forum/ prefix to permalinks. The setting recommends to be checked, but unchecked fixed my white pages issue.
