Trimble Quadri Help
This error needs to be fixed by Trimble Support:
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.