I installed the Gravatar Hovercards plugin, which works fine on one of the default themes, but seems to do 'nothing' in Athahualpa 3.5.3.

Somewhere I found that a problem could have something to do with jQuery, but as I know next to nothing apart from plain old xhtml and CSS, that doesn't really help me...

But maybe someone over here who has a solution?

I just tried on my test site and the plugin worked fine on Atahualpa 3.5.3 but I had to deactivate some plugins. I didn't dive into which one though.

It didn't help me all the way, but at least it gave me a clue:

I disabled all my plugins, and at first, nothing seemed to change.
Then I found out that sometimes at least the gravatar 'wiggled' while hovering, but didn't show anything...
And ultimately it became evident that indeed not much happened on most of the gravatar-icons I tried... because they all were in 'second- or third-order comments'. (Reactions on previous reactions).

However, in gravatars in 'first-order comments', the Hovercard did show... (I tried it on this page, where there is only one 'gravatar-accompanied' first-order comment, all other comments (mine) with a gravatar are second-order or deeper...

I thought my findings in my previous post gave me a solution: just disabling the nesting of posts, and all should work fine???
No, it doesn't: I tried unchecking 'Allow threaded (nested) comments...', on wp-admin/options-discussion.php, but as a result the plugin didn't work at all, not on previously nested comments, not on that previously 'first-order' comment.