RESOLVING THE PROBLEM
In a text editor, open the *.MXL file and look for the following tree:
<ImplicitRuleDef>
<UseConstant>
<ConstantID>8</ConstantID>
<ConditionalFieldID>188608</ConditionalFieldID>
</UseConstant>
</ImplicitRuleDef>
Change the value to any number under 32767. For example <ConditionalFieldID>32700</ConditionalFieldID>.
All the elements that are over that number need changed. But you should also keep in mind they should be unique.
Do not change the value to -1 though. This will also correct the issue and allow the map to be opened, but the fields in question will lose the standard rule “Use Constant”.