A loop occurs in the task group for node with NODE_ID &

What causes this issue?

One of the tasks in the task group refers to a subsequent task that
refers back to the original task, possibly several steps later.
This produces an endless loop.
Technical information: All tasks with the same PARENT_ID belong to the
same task group and are connected to one another in a chain. The content
of the NSIBLING_ID field refers to the NODE_ID node of the next task in
the chain.
If one of the following tasks with this NSIBLING_ID refers back to the
NODE_ID of the original task, an endless loop occurs and
the task list is faulty.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.