Looking good there Dirk. The chalky appearance of the matt clear coat is usually caused by the matting agent going into 'retirement' ! A good stir immediately before use should avoid this, even if stirred previously. This can easily happen with marked variations in temperature and humidity, especially so if sprayed.
If it happens again, then before re-coating in gloss and then re-applying the matt clear coat, just wet the affected surface with your finger (not water). If it goes clear again, then it'll clear - up with a gloss clear coat. If it remains chalky or milky, then it will probably need the top surface removing, by either very light sanding then a polish, or a rub over with a cloth only just dampened with the appropriate thinner.