Thanks for the report. We're aware that some Windows machines will crash when enabling Fast Draft in After Effects Beta. We're working on a solution for this, which I hope will be available in the next couple of days.
In the meantime, in the latest build of After Effects Beta (as of Feb 8, build 18.0x17), you can work around the problem by running After Effects Beta as an administrator. (Go to C:\Program Files\Adobe\Adobe After Effects (Beta)\Support Files, right-click on AfterFX.exe and choose Run as Adminstrator.) This step will, of course, not be necessary once we fix the bug.