Hello Ivo,
I just noticed that my fast boot option just stopped working, but only if I leave my PC off for an extended period of time (eg overnight or hours) and I am not sure if Classic Shell has anything to do with it but here is what I found out so far:
Checked Event Viewer and found this log:

Next, I also noticed that Classic Shell is not working (eg Start button returns to default Windows 8.1 operation).
So, to further investigate what issues I might be having with fast startup and the above log referencing an unknown wake timer, I input the following message. Note the response:

Legacy kernel driver? So, on a hunch, I uninstall Classic Shell 4.02, reboot, and reinsert the same message again:

Now all of this is way beyond my technical understanding; however, I just wanted to give you an FYI to see if any of the above makes sense to you. Anyway, I am running a clean install of Windows 8.1, using the latest GA ISO via download trick at eightforums.com and all available Windows Updates have been applied to current. THe only other issue I have had thus far is 5 app updates that failed to install (initially) from the Windows Store; however, I went back a day later and they all updated just fine. However, I don't care anything about apps or use any of them either.
So, now the BIG TEST will come tomorrow when I attempt to power on my PC again and see if my fast startup functionality has returned to normal. I'll report back at that time. Please understand that I am just throwing this out for your consideration to see if it rings any bells for you because I can't hear any.
Thanks,
njgroundhog