When you see this error or a permutation of this error -
CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY. The formula in the “SomeWorkflowOrValidationOrFomulaField” rule or process is invalid due to the following:
Division undefined: (System Code) External entry point
It does not have anything to do with your organization having the Divisions feature enabled but rather it's referring to a formula field involved somewhere within the configuration that is dividing a number by zero because the Blank Field Handling is set to "Treat blank fields as zeroes". So, for example, if you had a formula field which was used as the criteria for a workflow rule that specified 'FieldA / FieldB' and the value that was calculated for FieldB ended up as 0, at runtime this calculation would cause an error similar to the one above.