The raid controller swap did not have any effect on the problem
It seems that the second disk enclosure of the server is malfunctioning and needs to be replaced. In order to minimize the downtime, we will transfer a few disks over to another server (and migrate the clients there) and boot up the server with the enclosure that still works normally.
We hope that the server will be back online in a few hours. Thank you for your patience in this.
