When I try to quicksave via F5 it says in the top left corner, that it saved.
But ever so often when I try to load my quicksave via F9 it just says something like: "Can't find quicksave".

Then I have to load the latest manual save I made and all the previous quicksave-slots are gone.
My specs:
Win 7 64-bit
AMD FX 8350
8 GB RAM
Radeon R9 270x (2GB)

You can manual save but you can't quick save?

I can both, manual and quick save.
I press F5 and it shows me, that it made a quick save, but when I try to load the quicksave I just made it says, that there is no quicksave.
Not everytime, but very very often.

Check the save folder security permission. If everything is okay, check the quick save file security permission, 'Quicksave0_xxx..."

The Quick saving wording in game is misleading. No error will be shown even if it fails.