Error "couldnt_find_person"

I am trying to follow an account on Lemmy, but I get the error couldnt_find_person

I'm able to follow the account on Mastodon and some other Fediverse services.

The account is @bot@bot.viii.fi / https://bot.viii.fi/ - as far as I can tell, it complies with all ActivityPub specs.

Any idea why Lemmy instances can't follow it?