GENEVA — Switzerland’s biggest bank, UBS AG, posted a $2.1 billion loss for the fourth quarter Tuesday in the wake of a series of lawsuits and scandals and a wave of restructuring.

The bank, based in Zurich, attributed the loss mainly to ‘‘net charges for provisions for litigation, regulatory and similar matters as well as net restructuring charges and an own credit loss.’’ In contrast, UBS made a profit of $356 million in the fourth quarter of 2011 despite a $2 billion rogue trading scandal.