[kaiser]

An impeller that has come off the shaft or is corroded/worn away, could cause what is seen, but this is unlikely and very rare.
If there is an air pocket, then the water must have gone somewhere, and the system then has a leak!
That can be ascertained by checking the level once the car is cold. Press the top hose repeatedly and watch the level in the expansion box. If it sinks as you press, you are expelling air.
A total blockage of the system is also possible, but also rare. Maybe a problem with a hose de-laminating?

Of all the above, if circumstances are indeed as described, blocked passage caused by stuck thermostat is the most likely cause.