When in Project, if a new Project task is added or if tasks are moved, a generic error is seen: Unexpected error encountered. Contact System Administrator
Although this error can be generated for a variety of issues, the issues are typically data-related issues within the Project or Project Tasks themselves.
Generally, the first level of troubleshooting can be made by selecting Project Diagnostics in the Related Links section of the Project. *Available to any Project Managers
This will illuminate any invalid parents or incorrectly associated tasks. For example, if the top_task value is incorrect, or any parent value.
If the top_task is anything other than the expected Project (that is the top Project), you will need to address this.
If through Project Diagnostics or looking at a list view of Project or Project Tasks, an incorrect association is seen, you can correct this via the following methods:
*In situations where it is not possible to remove a field value (from security constraints, etc), you can always export the XML of the problem record (where the incorrect relation is), update the XML, and then re-import the XML of the Project task. This should only be done in extreme cases and should be tested in a sub-production instance first. This can easily be tested by exporting the Project from the problem instance (From the Project, navigate to the Project: Configure > Export to XML will export the entire Project and it's Project tasks)