Unable to take province

You will have to declare war with the inactive player in order to take the remaining province

You can take provinces only at war, other ways do not work.

(Or if you go trade with your ally or who ever you want.)

kill them all

neutral nation's armies are blocking you from taking towns thus you do not have really a choice.

it is due to a changes to a game that were made lately

bytro was trying to remove possibility of you attacking 2 people in a fort and taking fort after you defeated fort's owner

this behavior was very abused by better players and led to wining many battles where attacker should have died

It's a bad fix to an old problem. The old problem was that if you were defending an allies fort the enemy could attack the fort and take it out from under you and then you would be attacking him from outside the fort. They fixed this by making forts stay with the owner until all enemy or potential enemy units were gone. The correct fix would be to check the status of the attacking units against everyone inside a fort. If you're at peace or embargo with someone inside the fort then you must kill them to take it but if you have right of way or shared map with everyone left in the fort then you should capture the fort from the player you're at war with. This allows coalitions to defend their allies land against enemies (if you're not giving them right of way then you're forces will fight if the fort is attacked by a 3rd party) but does not allow an inactive former ally to block a fort indefinitely.