Henry - only one Core is allowed per Sooloos configuration. Make sure the MD600 is core and not the C15 and then reboot.

The Control is really a Windows XP embedded PC - sounds like if it doesn't boot the upgrade didn't complete or perhaps the kernel was corrupted. Hopefully that didn't happen and a power cycle fixes it.

Primary: 818v3+ID41, M6 stereo pair, M SpeakerLink cabling, Sonos+96kHz Cullen, silver DSW
HT: Marantz prepro, 271, DSP3300, 8K SE FL/FR pair, DSP M6x2, DSP320x4 (ceiling), black DSW, 520 RL/RR rear in-walls
Living Room: G65, 640 L/R in-walls, UHD722, MS600
Vac. Home: G61, HD621, 4xDSP3100, DSP3100HC
MDMS:NUC/ROCK+TrueNAS Core 12.0 ZFS file server