Thanks all for the replies.
After i did a 'zpool clear' a scrub was started. After it finished, the errors were still there and the pool still in a degraded state. The scrub had repaired 11G. Since i rebooted allready several times before the 'zpool clear' command, and i had tried to re-attach the (same) cables, i didn't expect a new reboot would make a difference, but i'm quite suprised, all the errors are gone and the pool is online...
The only change i made was in the bios of the lsi sas, from 'boot os only' to 'boot bios & os', which makes the drives initialize before starting the operating system i guess, but not sure. So i'm not sure if that made the difference.
I am however now not sure if the data repair by the scrub did not damage data? But since there were errors at that moment, i suppose what was corrected was right. I started a new scrub...
I don't think the memory is faulted, because i did not get any warnings via IPMI (or in the logs). And i ran a memtest also, but for sure not long enough. I do remember two months ago a disk from the same pool was once offline, a simple reboot solved it. This happened twice. After i installed a lsi sas controller (IT mode), so maybe the psu could be the cause? The system is build on a supermicro A1SRM-2558F.
I suppose i can not do much more at the moment but wait to see if i happens again or not. I had allready ordered a replacement drive, but i don't think something is wrong with the disk.
Any suggestions or ideas are still wellcome!
Thanks for the advise.