You aren't running Windows 8.1 through a virtual machine, are you? That was one potential cause of this issue. The other is the graphics card. Updating your drivers may fix the issue, or updating your version of directx. It's very odd that the game once started properly for you, there must have been something you'd done differently that made things work for you, but I can only guess what that was. Maybe it was a fresh install of the game, or you ran it with administrator privileges, or perhaps the filepath was different? Please let me know if any of these ideas fix the issue for you.
Yes, it is probably the same issue. I have been looking into it further and finding that it could also be that your graphics card isn't able to run webgl. You can try updating your graphics card, or, I've made a test patch that could fix the issue, it might not be that you can't run webgl, but that the game is preventing you from doing so due to risks of crashing. Doesn't really make much sense to make the game unplayable as an alternative though. Here is the link to the test patch. https://mega.nz/file/JegQVaTZ#efzK7M82djTOGtMG-4I82wfw0nVTqCWDmsYeVzB6FHc It's contents should be copied and pasted into the app.nw folder, if you are using the Mac version of the game.