I've gone through the .exe with a debugger and have found the problem. The problem isn't with Windows 8.1 per se - it is due to the number of fonts installed!
Due to shoddy programming, the Ever17 engine only looks at the first 100 TrueType fonts that it finds using the EnumFontFamilies() call. It checks the list to see if it contains a font that it knows about, but doesn't know when to stop checking! This causes it to eventually stumble into an illegal area of memory, causing it to crash.
One solution might be to start deleting fonts until the font that the engine wants (Courier New) comes up in the first 100 fonts found. Probably not a good idea.
I've also tried to skip the font check. This allows the game to run, but you get a weird font in the text. If you want to try this, use a hex editor (e.g. HxD) to open up the ever17PC_us.exe file and at offset 13337 hex, enter:
B8 01 00 00 00 EB 52
I don't have the time to look into this any further at the moment.