The description of the buckler (Player's Handbook 123, 124) (15 gp; 5 lbs.), in part, says
You can use a bow or crossbow without penalty while carrying [the buckler. I]f you use a weapon in your off hand, you don’t get the buckler’s AC bonus for the rest of the round. (124)
When a creature that's bearing a buckler makes one or more attacks with a light, heavy, or great crossbow that's wielded in two hands, does the creature lose the buckler's shield bonus to AC until the same initiative count on the next round?
Note that an answer may need to address the technical classification of crossbows as ranged weapons—and maybe even specifically as projectile weapons—rather than just treating them like melee weapons that fall into the usual categories of light, 1-handed, and 2-handed weapons.
Note: Avery Clamp is an artificer/crossbowman who wields a great crossbow on his turn but snaps it into his glove of the master strategist at his turn's end. (It's safer there—folks have to ready an action to make sunder attempts against his great crossbow. It's also really heavy!) Although he carries a buckler mainly for its additional abilities, Clamp would still like the shield bonus to AC from his +1 death ward durable wand chamber buckler after making his great crossbow attacks.