The only thing I could find in the logs is that the guest tried to use a VT-x feature not supported by the host CPU, but the host CPU DOES support VT-x and the guest is configured to copy the host CPU configuration so it should not use anything not supported by the host as it’s configured to use the same CPU.

     This is the first time I’ve seen this particular error and this configuration has been operating for ten years, so I’ve got to assume a random CPU or memory error.