We had a similar issue and dubbed it "white screen of death". We did not narrow the issue to Windows 10. It happened across many OS. The issue was intermittent yet often enough to cause us major grief.
I am happy to report that our issues disappeared since we upgraded to FileMaker Server 15. We have since noticed that web browsers regularly lose connection to the server under WebDirect, yet WebDirect consistently reconnects with few hitches. Version 14 was not so graceful.
Update to FMS15. It is a lot better performance and it have been better optimised for the different versions of web browsers.
Thank you all. I shall assess the impact of FMS15 on our current databases before deciding on the upgrade. Win10 was released after FMS14 after all.
In the meantime, our hardware staff also referred me this post -- https://community.filemaker.com/thread/152198. I have disabled Sophos on my test machine and it seems promising so far.