WMF Presto constantly fills the drip tray

Problem description:
The machine seems to fill the boiler frequently. This causes excess water to run into the drip tray.

Recently replaced the water pump at the bottom rear, since then error 166 has gone away.
Spare parts, service PIN and some experience are available.

Do you think replacing the expansion valve might help or are there other prime suspects. The machine does not make any error message. All components have been tested by driving from the service menu and are working fine. Last major service was after purchase a few weeks ago. My old Presto is currently being overhauled by me, but the drip tray was never full or we had to empty the broth mostly only once in the evening during maintenance.

The new one is an Aquaflex with tank. The other one only had the tank. But that should not really cause any problems.

The steam boiler sensor tube could be clogged due to limescale. This causes water to be directed into the drip tray through the safety valve when filling the steam boiler.