Using the base game, no DLC.

Used two mercenary armies to attack a different independent people's city in contemporary era; The siege initiates properly, units can be switched and moved as normal, but when I press End Deployment, nothing happens and the button can't be used again.

There's no support units in range, but one of my mercenary armies is reinforcements. There is also one reinforcement army from an allied empire.

Attacked another independent people's city right next to this one earlier, no problems there.

When I tried to make a save and reload, it gave me this error: The operation was canceled.

at Amplitude.Mercury.Game.Game+<DoStart>d__4.MoveNext () [0x00157] in <173fc3c1c3214bc89b88c03b179ae28b>:0
at Amplitude.Coroutine.Run () [0x00019] in <479babf06a1f4b9498857d4c8c52dc95>:0
Nubians Turn 344.ctr

Here's another save from earlier (City in question is Gwened to the very south):

quick-save.ctr