Caffeo CI

Hello, got me my 3rd Caffeo CI. She has only 5000 covers. Unfortunately, it shows the error "System Start" when preparing milk foam, which means according to Melitta: Air in the system, .... please vent, which the machine then also makes with pressure on the Rotary Switch. With the next milk foam, however, the same problem. I have now opened the machine - it really still looks top - leaks are not immediately visible. However, when foaming, you can see air bubbles forming in the hose. What could be the cause of this? Which part would have to be replaced?
Would be grateful for tips! VG

In which hose are there air bubbles?

see photo

What kind of machine is it exactly?

Does steam come out when you want to make milk foam? If so, does it also escape in the direction of the brewing chamber? There should normally be no steam or water in the short hose when you want to prepare milk foam, unless the outlet valve does not close properly or the line for steam is not free.
Coffee is brewing normally without the message "System Start"?

The machine in question is a Melitta Caffeo CI.
Coffee withdrawal runs without error message "System Start"; in my opinion, however, the amount of coffee is reduced. Incidentally, there is iA relatively large amount of water in the drip tray.
With milk foam, a lot of steam is produced away from the front, a short moment milk foam comes, then it spits, error message "System Start" occurs and Rotary switch flashes for venting.
I have filmed several videos, however they are too large to post here.
In my last post talking about air bubbles in the hoses, I had used water instead of milk to detect any leaks.
I thank you for your help!!!
VG

Who can give me a tip?

Have you ever decalcified the machine thoroughly? With an amidosulfonic acid-based descaler.

I will try, .... danke