You can't attack anything while riding a horse or other mount.
This is because the raytrace does not ignore the mount the player is riding.
https://github.com/SanAndreasP/BetterCombatRebirth/blob/master/src/main/java/bettercombat/mod/client/handler/EventHandlersClient.java#L190

Changing that line to
if(entity.canBeCollidedWith() && (entity != rvEntity.getRidingEntity())) {
will fix this bug.