The breadcrumb structured data is broken as demonstrated by Google's testing tool:

https://developers.google.com/structured-data/testing-tool/?url=https://area51.phpbb.com/phpBB/viewforum.php?f=81

The first two rows are missing its title and URL because itemprop="url" & itemprop="title" are not present.

To solve this itemprop="url" & itemprop="title need to be added to navbar_header.html @ line 186.