Same here.
New Z240 workstation with Xeon E3-1245 v5 and two monitors HP Z24n, both attached on the motherboard's DisplayPorts.
OS: Windows 7 Enterprise x64 Sp1 with latest patches and drivers (including monitors' one), BIOS (01.27 Rev.A)
I found two workarounds: power down and restart, or briefly attach another DVI monitor.
I will try your trick to power off a monitor.
P.S. Initially I had the same problem using two Samsung LS22A350N monitors and HP DisplayPort->VGA (D-Sub) adapters. I thought it was a problem with the signal conversion and changed monitors.