Paul Farber, executive vice president of 21st Century Insurance Group, resigned, becoming the latest defection from the Woodland Hills-based insurer. Farber, 48, who also held the position of chief operating officer, was leaving to "pursue other interests." He joined the company in 1984 and held various positions in the claims, marketing and operations areas of the company. Bruce Marlow, who was named president and chief executive last month, will assume Farber's operating responsibilities. Marlow replaced William Mellick, who suddenly resigned in January. Analysts said Mellick's departure signaled that insurance giant American International Group, which holds a majority stake in 21st Century and had expressed dissatisfaction with the way the company was performing, was strengthening its grip.