This error needs to be fixed by Trimble Support:

• Find the Location of a Workset

• Close the workset

• Press the three lines in the upper left corner of Quadri for Windows and select "Help", "Send to support"

• You only need to send the .gdtasks.sqlite file (or for older version: .gdtasks)

• When you receive a fixed gdtasks.sqlite file, replace your existing one and everything should be fine. Check if the task is at the right place, because it is random which parent/child connection gets fixed.