"Indexing Reference" Error

Notes on my experience with the "Indexing Reference" error, and how it was ultimately resolved.

• Been using RH since pre-Macromedia; managed multiple upgrades over the years/versions.
• Been using RH 2019 for approximately 1 year for 2 projects created in that version.
• Kept up with updates; currently running RH 2019.0.14 New
• Installed RH 2020, but holding off on upgrading/using to see what issues/bugs might be present.
• Also maintaining/actively developing 13 legacy projects in RH 2017.
• Started the upgrade for those legacy projects to RH 2019 (with the assumption we'll be going to RH 2020 once I've confirmed it's stable/supportable for OUR needs).
• Part of my upgrade process/test involves test-generating/compiling Responsive HTML 5 output (our main/primary output type).
• 12 of the 13  projects passed all my tests - successful upgrade and compile.
• 1 of the 13 projects successfully upgraded, but did not compile. Got stuck on the dreaded "Indexing Reference" process/error.
• Looked at everything related I've seen on here - broken links, image maps, TOCs, CSS, etc. Nothing I tried resolved the issue. 
• SUCCESS! Upgraded a copy of the RH 2019 Classic version of the project in RH 2020. Upgrade was a success and compile/generate was a success. 

So now I get to begin the test-upgrade process for my other 12 RH 2019 projects. Oh joy.