Interesting ... I did a look in the "A" slot from my G5 (7,3) and see
in there a Samsung EVO 850 with 120 GB (which should be a SATA III drive?!).

I remember that sometimes when starting the computer the "folder" sign appeared
(no disk found ?!) and I needed to restart. Then it worked.

The Samsung EVO 850 is confirmed to be working.
In my case it runs as described in slot "A"

So was there a SATA I or II version of this ssd drive, which should only work
in slot "A" ?

Maybe this ssd is somehow downwards compatible to the SATA I controller in there
and thats the reason ?