Just updated to the newest Vivaldi 2.8.1664.40, after that drag and drop tasks stopped working.
I don't know if it's because of WeekPlan or Vivaldi.
We can't reproduce this issue on the Vivaldi browser. It works fine for me on windows 10. Are you trying to drag and drop the repetitive task? The drag and drop feature for the repetitive task is disabled due to the restriction of Calendar sync. We will reenable one we found some workaround for it.
Customer support service by UserEcho