I just had the same issue come up. Here is the process I went through to both cause and fix the issue:
Went to: /wp-admin/admin.php?page=bp-page-settings > selected “New Page” to create the “Register” page > assigned “Register” as the page on /wp-admin/admin.php?page=bp-page-settings > At this point the page was there, but blank.
To Fix: Deleted the “Register” page and emptied trash > Created a page called “Register” from /wp-admin/edit.php?post_type=page (Add New) > Went back to wp-admin/admin.php?page=bp-page-settings > Assigned the “Register” page again. The registration info appeared.
There appears to be a bug in the Create “New Page” function (just a thought).
