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When using Host Device, the VM will not start with a host device serial port. We have tried reassigning the host port to a different port number, as well as differing formats for the port name - \\.\COM1 as well as COM1: and COM1 with no luck. Any idea what im missing here?

Serial ports can't be shared, so make sure nothing else is already using it.

If it's a USB serial then another no-no is unplugging the USB connector after the port is opened. Plugging it back in begins a new session, with a different device handle.

Already verified nothing else is using it, fresh reboot and nothing. Open putty and I can open the port. Its a local built in port (Dells version of a toughbook for industrial applications) so not USB and no unplugging possible.

After some more digging, found the user had installed a bitcoin miner as well as a few other things on the base machine, and one of the added applications was intermittently using the port.

I see, thanks for reporting back.

No problem, vanishing issue reports bug me on another OSS project i frequently contribute to so wasnt going to cause it here! Intermittent part threw me off for a bit for sure though...