P.S.: Just tested the current ME-cleaner version with the OEM BIOS version: The CPU fan did spin - even after hard resets.
So a) The issue is only related to coreboot. b) It is possible to do better here.
I gave it one last shot and externally re-flashed the coreboot image with cleaned ME and... well the CPU fan worked even after several hard resets.
At first I thought I'm stupid and then I tried the following:
Internally re-flash with the coreboot version with ME --> CPU fan worked as expected (I had ME after all).
Internally re-flash with the coreboot image with cleaned ME from above. --> After some hard resets the CPU fan stopped working as I had experienced it before.
Internally re-flash with the OEM BIOS version with cleaned ME. --> CPU fan worked again after some hard resets.
Externally re-flash with the coreboot version with cleaned ME without CMOs battery. --> CPU fan still worked after 3-4 hard resets. --> SUCCESS!
In short: Flashing the OEM version with cleaned ME *before* flashing coreboot seems to fix the issue. If you flash from the OEM version with ME, you'll run into the issue after some hard resets...
I don't really understand why, but I'll certainly enjoy the results! ... and a beer now. ^^