This is a generic error that can occur in a number of different situations when the transient results become too unstable. This may happen in some cases where extreme results are reached, such as very large air or vapor pocket formation.
First, check the initial conditions calculations for any issues. For example if you have pumps that should be on, make sure they have a reasonable operating point (not zero flow). If you are getting red user notifications in the initial conditions (Unbalanced Network, etc.), these would need to be resolved before computing the transient analysis.
In a model that ends at a Discharge To Atmosphere node, check to make sure that both the "Flow (typical)" and "Pressure drop (typical)" fields are populated. If one is left at zero, it can lead to an incorrect pressure vs. flow relationship, which can then cause problems with the initial conditions results and thus the transient simulation.
Check any surge tank configurations in the model for errors. If using the surge tank with a turbine, or in other cases where it should operate at "line" pressure, consider choosing "true" for "treat as junction?", to allow the initial, equilibrium HGL to be computed for the tank.
If this information doesn't help, knowing more about the model and where this occurs would be helpful. You can post a copy of the model, but if it is not something you want to be public, you can upload the model using the directions at the following forum post: communities.bentley.com/.../241343.aspx
If you do upload the model, let us know so we know that it is available.
Thank you.
Regards,
Scott Kampa
Bentley Technical Support