I had this problem recently with Channel 4 too. I think I might have found a fix - at least, it worked for me.
In Windows Explorer, go to C:\Users\{your username}\AppData\Local\CEF\User Data\WidevineCdm. If at any point you're asked to enable admin access, do so. Then try to open the folder in there called "4.10.2652.2" (version number might be different).
Again, you'll probably be prompted to allow admin access. Do that, then just run StreamFab again and see if ITVX/Channel 4 works now.