There's an issue where both the main "Forums" and "Topics" RSS feeds generated by wpForo don't validate, and it happens right here on these forums, too.
http://www.feedvalidator.org/check.cgi?url=https%3A%2F%2Fwpforo.com%2Fcommunity%2F%3Ftype%3Drss2%26forum%3Dg throws the error:
line 5, column 23: link must be a full and valid URL: # [help]
<link>#</link>
It's causing issues with Feedly and such, as well, likely due to the same reason.
Hi @davehamilton,
Thank you for letting us know, these issues are already fixed in next version. It comes very soon.
We updated our site to wpForo Forum 1.5.2, as I'm assuming you did, too, and this error still exists (referencing the feedvalidator link above).
Also, related, is this issue where the content of the RSS feeds contains HTML entities in raw text: https://www.macobserver.com/community/forum-suggestions/post-look-funny-when-viewed-via-reeder-3-rss-reader/