This may be related to https://github.com/xcp-ng/xcp/issues/282
My guess is that the tools ISO was mounted into the XO VM, then updates were applied to the host that contained an update of the tools ISO, which led to the XO VM having an non-existing tools ISO still mounted.
I have made a change in 8.1 to avoid that (eject all tools ISOs after updating the ISO, not just from shutdown VMs), and testing will tell us whether that's a good solution.
To solve the situation, you usually just need to eject the ISO with xe vm-cd-eject, not xe vm-cd-remove, as @tuxen said. If you had a snapshot of the VM before removing the CD drive, I think restoring it should bring the CD drive back and then you can eject the CD with xe vm-cd-eject... Unless restoring the snapshot also fails. Else as tuxen said you need to recreate a CD/DVD device and attach it to the VM.
iw0der created this issue in
xcp-ng/xcp
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This operation cannot be performed because the specified virtual disk could not be found
#282