It actually is a $BROWSER variable bug which xdg-open doesn't get. And xdg-open doesn't have midori in its list of popular browsers which it checks when no browser is set:
/usr/bin/xdg-open: line 406: xprop: not found
/usr/bin/xdg-open: line 436: firefox: not found
/usr/bin/xdg-open: line 436: google-chrome: not found
/usr/bin/xdg-open: line 436: mozilla: not found
/usr/bin/xdg-open: line 436: netscape: not found
xdg-open: no method available for opening 'http://thisisacensored.url'
EDIT: I fixed it by adding midori to xdg-open.sh. Probably not the best way to fix it but it seems like it doesn't get the $BROWSER variable.
Changed it to sth like this: "if no $BROWSER declared check midori first and then the other"