Strange CCD linearity curve

This is a linearity measurement of my Artemis4021 CCD. What is very strange is the jump halfway the graph....I am still very puzzled what that can be. I have it with Nebulosity2 and ArtemisCapture capturing software...
After consulting some fellow-astronomers on the internet I decided to put extra paper between the EL panel and the CCD sensor, thus increasing the exposure time drastically. What is very clear, is that I have to stay away from the 2.30sec exposure time, because from that point on, the sensor is almost linear. See the new curve I made with longer exposures:

This is more like it. I am very happy with it.