We have this saying where I live, which goes something like: "...or you plug it in". What is says is that sometimes the solutions to a problem (especially with appliances) are just around the corner after you have searched for who knows how long in the wrong direction (meaning the appliance wasn't working just because you forgot to plug it in).

So: I have taken the flow restrictor out of the upper tube and put it in the lower one. And now everything is cool!

The importer told me I should put the restrictor in the upper tube, but he was clearly wrong. It seems like the restrictor was pushed out of the tube by the incoming water because it was not stopped by the grouphead. In the lower tube it stays put.
Now my machine warms up slower, but it idles at 93-95 degrees C. The orifice is a very small 1,5 mm max.(couldn't measure it properly). With Erics grouphead Thermometer on the way, I will be able to be very precise on temperature very soon. At least my joy is back!

Thanks for thinking with me!