On Instance1, whenever the form is loaded or a field value is changed, an unexpected popup message is displayed. This popup message is not being displayed for the same form on a different instance.
The popup message on the form was being generated by a client script for the f orm having the issue, which was different from the same client script on the other instance. The popup message was being generated due to a statement in the Script section of the Client Script.
Having the Script value of the client script to match the value from the other instance, resolved the issue of the unexpected popup message