Mark Cuban is suing the United Football League for allegedly stiffing him on a $5 million loan -- the first big sign the minor football organization you never knew a damn thing about ... might go belly up.
Cuban's HDNet filed the federal lawsuit this week ... claiming the UFL -- an upstart league of former and undrafted NFL players -- failed to repay the $5 mil it was loaned last April.
In the docs, HDNet says the league missed two deadlines -- in October and December -- to pay back the cash, plus interest.
A league rep said they don't comment on business matters.
The UFL started in 2009 and its games air on HDNet ... for now.