@Joulinar thank you for your hints.
The update now went without a hitch, only the system time is lagging behind by about 4 minutes per hour.
Having issues with your DietPi installation or found a bug? Post it here.
if you switch to hourly update, it shouldn't be an issue. But still strange that your time is not synchron. On my RPi3B+ running dev branch it is still in sync after hours. Are you running some CPU intensiv operation on that board?
Pls let us know if a solution is working. This could help others if they hit by similar situation. Your DietPi Team