I was able to solve it. I put togheter all the facts and came to a conclusion it has to be something really simple. Googled for yet another hour and a forum thread where LSI owners were stuck with the problem that they cant turn on disk cache and therefore can't use SSD-s catched my attention.
For regular HDD-s I don't use it, so this was the safe setting I was using for SSD-s also. I don't know how it worked without disk cache for the first 1,5 months but it has been without any problems after I created a new SSD RAID10 array with disk cache turned on.