Server stable again Wed, Jun 8. 2016
I'm pleased to announce my server is running without freezes again!
I finally took the decision of buying a SATA controller and swapping out the SAS controller. I've also started the Samba daemon again. That was one and a half weeks ago. Since then no freezes. I'm officially declaring my server to be stable again!
My guess the SAS controller (or one or more of the cables, but I think the chance of that being the culprit is lower) was causing the problems was apparently correct. In hindsight, buying the thing was a costly mistake in the first place: close to five times more expensive than the SATA controller and vague errors because the SATA disks are communicating with a SAS controller (so a different protocol). Yes, the SAS controller can accomodate 8 disks and the SATA controller only 4, but I still have two ports free, partly using the SATA ports on the motherboard, so no problem there. I've learned an important lesson and will never buy a SAS controller again when using SATA disks!