So after about a year that this problem occured relatively rarely, in the last few weeks Disk 1 (the same disk) disappears every few days and even several times a day (until restart), so now I'm worried and looking into it more deeply.
I switched both the SATA and power cables of Disk 1 with another disk in the array to see if the problem recurs with another disk. To my surprise, today the problem occurred again with the exact same disk - Disk 1.
The log is the similiar to the log JorgeB referred above (this time with ata6) -
Since it always happens with the disk, even with different SATA and power connectors, does that mean there is a problem with the disk itself?
or maybe 7 disks are too much for the MB/PSU (still unclear why it's always the same disk).
should I replace it and activate warranty in this case? other than disappearing, the disk is healthy.