Some dates get interpreted wrong or even throw an error - please see https://www.mediawiki.org/w/index.php?title=Project:Sandbox&oldid=2001499 for an example. How to fix this?
PS: This also happens on a wiki with language "German".
Topic on Project:Support desk
As it appears to me this seems to be something for a bug report on phabricator for the ParserFunctions extension.
I just figured out that it's not a bug. It's just the way #time works - the function accepts only english month names, not german ones.
To fix this I used |?Date#ISO in my SMW ask query.
This way the date is not given out as German (21 Juni 2014), but as 2014-06-21 - which then can be put into #time without any issues :)