U-791
is listed at ubot.net. You just have to know where to look.
http://uboat.net/boats/nc_791.html
Also
http://www.navypedia.org/ships/germany/ger_ss_v300.htm
It would seem, however, that the mysterious 'UX-791' never existed. I can't find any reference to that number other than these reports, which seem to be a hoax.