I became a vassal of one nation A, and when that nation A declared war on another nation B, I also entered into a warring state with nation B. However, when wanting to move into nation B's territory, I cannot, and I am told "Cannot be done as you have to declare war and don't have enough war support for that", even though I am already at war with them.

A screenshot of my screen at the time for any (possibly relevant) details to be gleaned.