After successfully agreeing/commenting to various posts, I got the “Something has gone badly wrong” message at this post: https://www.ispotnature.org/communities/uk-and-ireland/view/observation/805586/ladybird,
I cleared the cache, and re-started Firefox. Now I see this message:
I can, however, add a comment to the post.
Updated: however, on this one,https://www.ispotnature.org/communities/uk-and-ireland/view/observation/805576/yellow-and-black-caterpillar
I can agree, but not add a comment: