Function: feeds.attachments_base.fetch_attachments
The sql query pulls all attachments from the attachments table. For phpBB installations with large attachment tables, this causes memory overload/server 500 issues.
The sql query needs to be time boxed based on the a.filetime of the oldest post.post_time.
For my temporary work around:
https://www.phpbb.com/community/viewtopic.php?f=466&t=2390801&p=14544781#p14544781