I just did another test with a freshly mixed solution (this time 1+10 instead of the normal 1+14) which didn't gave me better results.

What suprises me is that - apart from the lack of contrast - the developer works just fine. The image on Adox MCP begins to show after 10 sec. and the blacks are as deep as they should be. I will pick up a bag of Dektol on monday to be sure it's not human error...