Ok, so I have been able to solve the noted issue by doing the following (though I am not sure why this resolved it):
1. Changed all ‘URL’ field types to ‘text’, saved custom fields.
2. Changed all [applicable] ‘text’ field types back to ‘URL’, saved custom fields.
3. Updated option page fields accordingly, and clicked “Save Options”… And everything saved — no JS error.
If anyone runs into a similar problem, maybe the above will help.
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