Extremely high calculated values of air speed through a vent location can cause problems in the analysis.
Edit the venting analyis location properties to increase the vent size, or set more venting analysis locations on the model before starting the analysis again.
To change properties of existing venting analysis locations:
To set a new venting analysis location, select (Home tab > Molding Process Setup panel > Boundary Conditions > Venting > Venting Locations) to begin.