I happened to notice that some text was missing at the bottom of the advanced search page. Turns out that it was a problem with a language string in search.tpl:

Code
<label for="bodyprev">{$lang.PREVIEW_BODY}</label>
should be:

Code
<label for="bodyprev">{$lang.PREVIEW}</label>
(Not sure if this is an actual bug or if something went wrong in the upgrading process.)