Gentlemen,
thank you for your responses. First, I checked my schema and all the fields permit null values. Not really sure how to use the developer tools to trap errors while I'm editing a point feature in my web map. Maybe a good reference link could be provided.
Also, my feature template, while editing the feature in ArcMap seems to be ok, as far as I can see. The feature does have a template and all fields, including the three fields that share the same domain, all permit the null value to be stored.
I will attempt to publish the feature service again to see if my templates are mixed up.
Walter