+ AA6YQ comments below
This has happened twice now, so not a one-off event.
I tried to turn the Yaesu rotor using DXView and it didn't respond. I manually tuned it and it worked fine. The rotor is a Yaesu G-450A with the aftermarket ERC interface.
I opened the DXView config and unchecked the enabled box. After a few moments DXV stopped responding. Task Manager can't kill it, even the details tab showing processes can't kill it. TM shows it using 120k of memory, the CPU is at 15% utilization.
I tried the ERC standalone tool but the Com port is locked in use by DXV. Still, the interface has been working fine with N1MM Rotor.
Not sure what is happening, but a re-boot seems like the only way out of this.
+ That's most likely a defective COM port or COM port driver.