Using howardgmac's suggestion of setting a policy to run weekly (or
daily, if you like) to remove /Library/Bundles/OSXNotification.bundle
has removed the Mojave upgrade notifications as well as keeping the High
Sierra notifications away on Sierra Mac...
Saw the same error message on a 10.12.6 Mac this morning that turned out
to be an erroneous subnet mask. Verified that the mask that worked was
proper by our network team, and updated the documentation.