Well, I realized that this might be useful to someone, but it is not perfect.
First, take the aspx page code and paste it into another file (as a backup). Then uninstall WebPart on WebPart , then WebPartZones , and then any other possible server-side markup, until the page stops breaking. That way, you at least know where the error is.
For me this time was WebPartZone , which had two attributes together, with no spaces between them. Such errors can be very complex.
I never do this crazy barbaric hack on the sharepoint aspx page again.
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