This is the human-readable text accompanying the remote-invocation error code for the master extension key UCS generates to sign/authenticate the VM-FEX control channel to the VMM. Where the numeric code only tells you a remote call for the master extension key failed, this string is what actually explains why — often forwarded almost verbatim from the remote system's own error response. A typical value might read 'Unable to connect: connection refused', which for the master extension key usually means the far-end management service was unreachable at that moment.