Not sure exactly what fixed it, but it could be 1 of 2 things:
1 - I tried to confuse the monitor even further by disconnecting and reconnecting the signal cable and power cable in all the possible sequences that I could think off, while the computer was on.
2 - Together with 1 above I also bypassed the HDMI adapter (HDMI @ computer and DVI @ monitor) by connecting the monitor directly to the computer with a DVI cable. (my graphics card has 2 HDMI and 1 DVI socket) (monitor set to auto source)
The above appeared to not have solved the problem, so I then reconnected the HDMI adapter. When I powered down again, the monitor went into sleep. (now i'm not touching it again - ever)