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Thumbnails broke again yesterday (7/11/18), apparently from a Windows 10 update.
In Acrobat Preferences | General, "Enable PDF thumbnail previews in Windows Explorer" was checked, but didn't help.
I clicked "Set as default PDF handler" and went through the process. It said Edge was the PDF handler, even though PDFs were being opened by Acrobat. Choosing Acrobat restored the thumbnails.
Seemingly, the Windows update snuck Edge in as the icon generator, even as Acrobat Pro 2017 continued to be the default for opening PDF files.
My version is now Acrobat Pro 2017.011.30096.
It's not fixed, Walt. Please remove the erroneous claim that it has been, and provide a workable solution.