Here's the latest data (from october 20th to be honest):

Until about t = 330 s everything is fine, the galden is slowly heated, starts to evaporate and finally the chamber is filled with saturated vapor. As soon as excess galden flowing through the chiller reaches the chiller outlet thermocouple, the temperature at that location suddenly rises. The safety code kicks in and disables heating. Another very important thing happens now:

During the heating phase, small amounts of galden vapor condense in the chiller inlet and build a liquid drop in the teflon tube. When more galden vapor enters the tube, this liquid (now cooled down by the ambient air around the tube) is pushed back into the chamber and suddenly cools it down. This can happen for two reasons:

• The chiller inlet tube goes upward from the chamber to the chiller. Any galden condensing in the tube will flow back towards the chamber.
• The fitting between the chamber and the chiller inlet tube has a small inner diameter (about 2 mm I'd say). This prevents the galden from actually flowing back into the chamber, so it can block the tube instead.

After that the chiller outlet is slightly warm, and the control code can start saturating the chamber again.

This intermittent flushing with cool, liquid galden prevents the oven from operating how I intended it to. But it does solder PCBs, so I'll call it good.