I think lemmy.ml has more posts, comments, likes in the database due to historic reasons - and the backend is timing out, PostgreSQL not keeping up to application timeout. I have seen the problem on Beehaw, Lemmy.world when they are busy and throwing nginx 500 errors on their front page.

I’ve also been seeing issues with subbing to lemmy.world communities from lemmy.ml, so I am not sure if it’s limited to just lemmy.ml communities.