After a lot of reasearch and sweating I finnaly figured out the issue. This issue was driving me nuts.
Looks like it was the firmwire version on the LSI SAS 9211-8i controller. The controller was flashed with version P20. This firmwire version can cause a lot of troubles like drives falling out of the raid, CRC Errors etc. I flashed the controller back to firmwire version P19, reconnected my drives, and tried some file copies on the drive pool. None of the drives increased the error count. It's a shame the errors will always stay visible in the S.M.A.R.T. details, but I'm glad this fixed the issue.