Error firing vbld_BuildStarting event

[bwilder]

Hi,
Suddenly, today, a script of mine that has been running successfully for months failed to run with the following error in the output panel:

Error firing vbld_BuildStarting event: Error in Project (VBScript) script code at Line 51, Column 6
Build ended.

The build didn't even start, just sat there for a few seconds and eventually dumped out the error. The issue persists even when using backup copies of the scripts that are weeks or months old, so I doubt that it's the actual script that is causing the issue. Something must've changed with the environment, but I'm not sure how to proceed.

Where is the vbld_BuildStarting event located?
What VBScript should I look at to troubleshoot this?

Visual Build Professional Version 7.2a