Apparently it is not a good idea to use special characters in forum tags. It is natural to choose "c++11" as a tag when the topic is around the specific standard. The flag has been used in a couple of posts. However, they are not accessible through the tags search. Respectively, they are available and you can choose them, but the subsequent does not report any result. This is not the only tag, but also the others with plus sign in tag.
Just click on "c++11" used in forum post http://forum.qt.io/topic/56608/some-tags-cannot-be-used-for-search as tag. Also there is an empty page.