Comment 1 for bug 700283

Somehow you have told enblend to use the command line argument "-U" which doesn't exist. Valid arguments are described here:
http://wiki.panotools.org/Enblend_reference_manual#Common_Options

There are two places in hugin where you can specify these arguments: General behaviour for all new projects via the preferences window (see http://wiki.panotools.org/Hugin_Preferences#Enblend for useful options); or specific arguments for the project you are just working on via the "Stitching" tab (press enblend's "option" button).

Just exchange the -U argument with something more useful (i.e. '-v' will output some extra information while enblend is merging your images) and stitch again.