The following URL was failing:
It was determined that the correct URL to this form should be:
The binding for the K2 Runtime site URL was pointed toward an older binding which was no longer valid (K2.something.denallix.com); instead of the currently valid one (k2.denallix.com).
The value in the hidden 'K2 Settings' list for this site is correct.
To resolve this issue, navigate to [K2 Application > Settings > Form Settings] for this list and perform the 'Save' option. This will update the appropriate URLs to the currently correct bindings.