So what was actually wrong?
What was the problem with the expansion tank?

Not all pressure tanks have a bladder. The air in the tank can then
dissolve into the water and go somewhere else (radiators?). May have to
drain the expansion tank occasionally. The one I have - valve tank off
from the heating system, open the tank drain valve, I think if the screw
on the valve handle is removed it allows air into a very small pipe to
the top of the tank which allows draining better.)

There is a device on some systems that removes air from the circulating
water and sends it to the expansion tank.

Some systems have a valve from the house water system that auto-fills
the heating system to a set pressure. There has to be a check valve
between the house system and the heating system.

It was filled with water. I think the bladder must have been broken. From the looks of the old expansion tank I think it was older than the boiler. So well over 20 years old.