both are previews (betas) this may occur, I think you're at risk relying on previews if it's your main rig.
I am using Fs Protection on the main system windows 8.1, he is acting well, I have Windows 10 on the virtual machine and rather too early in order to test anti-viruses of any companies on it. But it is only my sentence.
As a developer and enthusiast, it's important to be prepared for the next versions of Operating Systems, so, at home, on one of the computers, I do use the latest TPs or betas to see what's new. I don't mind my machine being unstable or having some incompatible appplications or hardware for now (even though it did not happen to win 7 or win 10 previews or betas). So, I am ready to take this "risk", although I haven't seen any serious errors or BSODs with W10 TPs. What a different risk is having no internet and virus security protections, since then it creates a risk of network drives and other computers being accessed from my computer.
Can you guess how many people are running TP of Windows 10? A pretty large number, I would say. So, some of them who were using F-Secure solution before, now need to look for the "temporary" alternatives. But then, who knows.
My guess is that it would not take a long time to fix the compatability issues with TP9926. It might be also good for the time when the RTM version of Windows 10 comes out and security solutions are ready even before the RTM is released. However for now, I tried IS and Freedome and neither of them work on 9926.