Hmm - not sure. What are you running it on? The 52840DK, or the dongle?
The board file was for the DK so perhaps there's something different in the pin mappings? <- Serial1 happens when the REPL moves to Serial1, which I guess it might do if USB dropped out? The code to swap REPL probably isn't designed for working with USB, Bluetooth and serial so it might choose the wrong one.
It's possible that the computer you're using keeps connecting & disconnecting on USB, which causes that?
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