From the Kaeser manual I find that "fan M4 I" error means 'Third fan motor shut down because of overload'.

So form your question I understand that this happened the first time.. but now the alarm keeps coming back without M4 actually tripping? Right?

It sounds like there's a bad contact in the M4 overload relay, or in the wiring or other parts in the alarm system.

But is suspect the overload relay more, because the problems started happening after it tripped once.

It's hard to find.. because if you check afterwards with a multi-meter, often everything is fine.

If the contacts bounce open for only a few milliseconds, then it will already generate an alarm.

They could bounce open because the machine switches from unload to load and the makes the machine vibrate a little bit.

Overload relays are very susceptible to this kind of thing in my experience, especially when they have tripped before and are already some years old..

So please check that first.. if you get this alarm every time the machine starts, you can try to short-circuit the wires on the trip contact and see what happens. But you gotta be sure that the fan is OK, because you by-pass the overload safety. just try it for testing...

Hope this helps and love to hear how it works out!

Cas