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@Chemist1 - what do you have as the Windows default program for opening PDFs, which is what calibre is trying to use?
Might be an idea to reset it.
This has worked for me a couple of times when a Windows default has gone awry. Set to something else - e.g. Edge, restart (yes I know its a PITA), and then set it back to what you want - e.g. Acrobat.
BR
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