@oliver
There's a preliminary check that Friendica performs when searching by URL: it sends a short test request (HEAD) and decides based on the response format whether fetching the post is worthwhile. This exact check fails on lemmy.ca. The instance responds with a 404 and no format information at all. It could be caused by a reverse proxy in front of the server, or by other firewall/protection systems.

lemmy.today, by contrast, cleanly responds with "200 and ActivityPub format".

As a result, Friendica reports "no results", even though fetching the post itself works fine.