A close relative of Toni Braxton tells TMZ ... the singer's financial undoing can be traced back to her show in Las Vegas.
The relative tells us Toni took out "show insurance" through Lloyd's of London for her act at the Flamingo Las Vegas Hotel & Casino back in 2006.
And when Toni's heart condition started to act up -- in the middle of the show's run -- she had to pull the plug, but she was still on the hook to the hotel for big bucks. Toni expected Lloyd's to cover the loss but Lloyd's didn't, arguing Toni had an undisclosed, preexisting medical condition, rendering the policy void.
In the end, Toni had to absorb a huge loss, which the relative claims sent her on a downward financial spiral.
Still, it doesn't explain the bill for the Four Seasons and Tiffany & Co., for starters ...