The yolink module I maintain hasn't been getting along with Fusion themes, it seems. Both of them wanted to control search-result.tpl.php and search-block-form.tpl.php. My understanding is that the theme should win the fight and search results and forms should show as Fusion wants them to. That would be okay. But what actually happened was that no code from either of the two files showed up. A week of Googling and troubleshooting later, it looks like I have this problem -- either there's something incomplete in fusion_preprocess_search_result(s?) or there's a bug in how the double-template situation is handled. It is interesting that the response to this possibly related error seems to have been to silence the error messages.
I don't have more time to spend on this now, so as a workaround, I've ceded control of the two conflicting files, and yolink's back in action on Fusion, whose subtheme Acquia Marina you see before you this week.