It looks like it should work; I can pull a search:

http://de.wikipedia.org/w/api.php?action=opensearch&format=jsonfm&search=0

I see some funky JS errors in firebug when trying to pull the '0' like:

box.getNotificationWithValue is not a function
http://de.wikipedia.org/skins-1.5/common/mwsuggest.js?195xx
Line 125

os_decodeValue("0,5%-Hürde")mwsugges....js?195xx (line 125)
os_createResultTable(Object searchform=searchform searchbox=searchInput, ["0", "0 v. Chr.", "0'00''", 7 more... 0=0 1=0 v. Chr. 2=0'00'' 3=0,5%-Hürde 4=0,5-%-Hürde])mwsugges....js?195xx (line 368)
os_updateResults(Object searchform=searchform searchbox=searchInput, "0", "["0",["0","0 v. Chr.","0'00''","0,5%-H\u00fcrde","0,5-%-H\u00fcrde","0-0","0-1-Gesetz","0-8-15","0-km-Stein","0. Dynastie"]]", "http://de.wikipedia.org/w/api.php?action=opensearch&search=0&namespace=0|4|11")mwsugges....js?195xx (line 344)
os_updateIfRelevant(Object searchform=searchform searchbox=searchInput, "0", "["0",["0","0 v. Chr.","0'00''","0,5%-H\u00fcrde","0,5-%-H\u00fcrde","0-0","0-1-Gesetz","0-8-15","0-km-Stein","0. Dynastie"]]", "http://de.wikipedia.org/w/api.php?action=opensearch&search=0&namespace=0|4|11")mwsugges....js?195xx (line 401)
onreadystatechange()()mwsugges....js?195xx (line 428)
[Break on this error] return decodeURIComponent(value);

It's a little hard to follow, but I think the suggestion table generator is trying to URL-decode the results, which don't appear to be URL-encoded. When it hits the % sign in "0,5-%-Hürde" the URL decoding fails? And then firebug also spits out a totally unrelated error about a null or something. o_O

The search appears to be good, indeed.

That looks like either debugging the JS-stuff,
or simply coding the % as \0025 so as to avoid
the "stupid" urldecoding attempt.

Marking as worksforme. Search front-end has changed a lot in two years, if the bug is stil there (I dont see it on de.wikipedia, '0' returns a few page names correctly), please re-open.