While you don't provide enough info to know for sure, it sure sounds like this error is being generated by vc6 / devenv.
If you check the Show command-line ... option on the Options tab of the Make VC6 step, then rerun the step, the command-line that Visual Build Pro uses to invoke the vc6 compiler will be logged.
Execute this command-line directly from a Command Prompt and see if the same error is generated.
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