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Assuming the details (comments) section of Calibre shows valid formats.
Probably, your device driver settings got corrupted.
With the device connected, click the arrow on the Device icon.
Make sure there are ticks next to the formats your device can handle (EPUB, KEPUB...)
Save.
If the ticks were there that match the formats that Calibre shows in the DETAILS pane. Run Library Maintenance , you might find that they are no longer present,because someone.something went into the Library folder and changed, moved, deleted them.
Rogue A/V or leaving Cloud sync while changing metadata, can clash. Pleas read the FAQ at the Calibre help site.
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