Hello Hans,
The only thing I know of changing that value dynamically is TLP, do you perhaps have TLP installed?
Indeed - thanks for the hint! I've remove tlp and now the system works stable with AHCI SATA mode again. I installed tlp because I read somewhere it can be used to read the number of charge/discharce cycles of a loptop battery.
I'll give F28 a try and see whether the changes there work fine for my machine - otherwise I'll file a bug report against the kernel.
Best regards, Clemens