Search Suggest doesn't display any results on a subwiki

From a user on the list:

http://xwiki.475771.n2.nabble.com/file/n7588155/SOLR_Search_Suggest_Failure.png

Instead of a full Search Suggest window, only the upper border is displayed.
I have this issue now on my XWiki 5.2.1, whereas SearchSuggest works fine on
my 5.1 install, which I've used for testing purposes.

I happened to notice that the same thing happens in the search field on the
XWiki extensions site (http://extensions.xwiki.org/xwiki/). In fact, the
attached pic is from that site.

I've tried all the configuration tricks I could think of, but to no effect.
I'm using IE10 & IE11, Windows 7.