I have had the same spinning wheel problem with my C15 on both the C15 Config screen and PC Control but it only happened after a Sooloss software version update. This was with a new SSD installed which I mirrored from the original 500Gb hard disk. When I put the original hard disk back in with the previous Sooloos version, everything worked fine.
I then mirrored the drive again and the problem went away. The update happened again but this time the spinning wheel problem did not appear. However another one did ! Now if my C15 goes to sleep, I can't wake it with a touch of the screen or App to wake it up. I got tired of rebooting it so now I just leave the screen on continuously.
My conclusion is both of these problems are software related as opposed to hardware. If someone could lend you a working Sooloos hard disk or SSD, you could test that theory by installing it into your C15 and see if the problem is still there or magically goes away. Unfortunately I am too far away to let you try my spare mirrored disk.
#1: 8000s, 5500HC, 8000s, 861v8, 800Dv4, HD621
#2: 6000s, 6000C, 5000s, 6000s, 568.2mm, 598DP, 511
#3: 5500s, 5000C, 5000s, 561, 800v3, 518