Comment # 14 on bug 979265 from Wolfgang Bauer

(In reply to Wolfgang Bauer from comment #13)
> 'caption Krusader --icon krusader_user' is indeed not a valid command line
> option, but I'm not sure yet why the whole is treated as one option...

The problem seems to be in the search_desktop_file() function in xdg-open:
  if echo "$arguments" | grep -iq '%[fFuU]' ; then
                echo START "$command_exec" "$arguments_exec"
                eval "'$command_exec'" "'$arguments_exec'"
            else
                echo START "$command_exec" "$arguments_exec" "$arg"
                eval "'$command_exec'" "'$arguments_exec'" "'$arg'"
            fi

That double quoting in the "eval" lines looks a bit strange to me, and indeed
changing it to eval "$command_exec" (and so on) fixes the problem.

OTOH, this has been added as security fix, see bnc#913676.

CC'ing the xdg-utils bugowner for further insight.

Btw, the function has been completely rewritten in upstream git meanwhile, I
haven't tried yet whether this problem still persists though.