After the disappointment with the "Liquimaster Pro" I gave it another try to mill the same part.
This time the workpiece was a little big bigger so I could clamp it better to the T-nut table.

I applied the coolant manually and again everything looked very promising. As before I milled very slowly and as everything seemed to work fine I went away to do something else.

But as you can see here, the same issue again:

As soon as the mill bit milled the last thin layer, the material caught by the flutes and lifted the workpiece.
And again the bit took some chunks and broke.

This is not as bad as my first try, but still a total failure.

So the root cause of all this is that you cannot clamp down anything really on this mill as the T-nut table is not ridged at all.
As you can see the thin, hollow profiles have no support over the whole distance form the front to the back of the frame. They are just screwed to the frame in the fron and the back of the machine.
You can wobble the profile with just a little force.

The next issue with those profiles is that they bended when I clamped something with a little force.

So I decided to change something.

A T-nut plate of massive aluminum in the size of the machine costs between 350-500€. That is quite a lot, but don't ask for one made out of steel. You would be surprised how much they charge you for a used one from an old mill.
So I thought a massive 10mm steel plate would be the best as table and then put something on top of it.
I managed to find a supplier that agreed to laser cut a 700x490x10mm steel plate for me and charged my around 90€ including the shipping. As a side-note, they payed the delivery service for 8kg, but the plate is more like 30kg ;-).
I also found a cut off end from a massive aluminium profile in 250x400x20mm for 40€ that I am going to mount on that steel plate. If this will work I might think about covering the whole area with a t-nut profile in the right size.

The next step is to mark the holes and drill holes in the steel plate that allow me to mount the steel plate with new M4 countersunk screw to the frame. I am using the same holes that where use to mount the aluminum extrusions. I was thinking to re-drill and re-tap the holes in the frame to M5, but I don't think that is really necessary. If it turns our to be, I can still do that later.