We have created a case at Adaptec.
They are current checking the logfiles.
I'm not sure, but current I would rather name the Controller the root cause, or the combination mainboard <-> controller.
sorry for the delay, but it was really a hard way :-(
We have replaced the Adaptec Controller - w/o success
We have replaced all Harddisk, even with different models - w/o success
We have tested only a pair of two disk (disk 0&1, disk 2&3, disk 4&5...) and always the last drive failed
Of course, every test took between 4 and 20 hours.
Finally, we have replaced the backplane (or better saying the whole server case).
And now everything works absolute fine.
We have used the same server case with a different controller but the same harddisk - w/o any problems.
So it seems it was a combination between the backplane and the controller.
Nevertheless, now everything is working, and we can finally start with our real tests.
To all open-e employees: Thanks you very much for your fast and amazing support.
Even it was NOT the fault of your software, your support was really great.
Good to know that there is somebody when something goes wrong
THANKS !!!