Browsing to https://www.phpbb.com/community/viewtopic.php?p=13747681#p13747681 I have the link "View idea at", pointing to https://www.phpbb.com/ideas/idea.php?id=1101, but that request is not served. Did you guys really ripped off your custom "ideas" MOD? Is it related to 3.0.x's EOL?

Bonus: the landing page says "Error 404: Not Found
Oopsies… what you have requested is gone!", but that is contradicting - either send HTTP status 410 if you know is gone, or output a more appropriate text, i.e. "currently not available".