What are the many suggested solutions you've tried?
Without exact system info and details about otehr running processes nobody can tell you much.
So i have an 8 core, Intel i7-4770 @3.40GHz, 32 GB RAM, 2 SSD, one for the system 120 GB and one for work 500 GB.
the videocard is NVIDIA GeFOrce GTX 960.
Actually i realized this preview problem since i installed AE CC 2017, i don't remember having this problem before.
Of, i purged the memory and disk cache, i moved the cache folders on the bigger disk, i've uninstalled the antivirus (learning that it worked in a similar case posted on creativecow forum), i cleaned 20 GB on the system disk
I guess something is forcing AE to use just 1 GB. In Device Manager i don't have any conflict.
i forgot to mention it's a Windows 10 system.
Sounds odd, though I'm inclined to think that some driver is pigeonholing you into this situation. Just to test I would create a secondary user profile and see what the differences are. Also try booting up in safe mode. Disabling devices and CoDecs in the device manager and then re-enabling them may also help to track down the culprit.
You have 0 GB of RAM reserved for other applications? That's going to cause lots of issues. AE even uses system resources outside of itself to do stuff. The recommended practice is to allocate about a quarter of your RAM to other applications.