To catch some attention, maybe list your config in your post and make a nice subject like "I have a really nice PC, game crashes." or something . Chances are you might actually talk to someone on the dev team if you're lucky and they reply to your thread.
The only thing I can think of is - you have the latest gpu drivers and a 64-bit OS right?
Definatly go to the forums, Tripwire is very good at working stuff out with its players.
Beyond that a few things you can try
1. Run the file directly from its folder, not the shortcut
2. Try running as an ADMIN
3. Possible worst case scenaro fix FULL wipe....(will delete all your PERK lvling info) Im not sure how much you have leveled. Though Tripwire said perk wipes will happen eventually. Better to play and relevel then not at all if it work. Maybe backup KF files before deleting them just incase.
1. Uninstall Killing Floor
2. Delete Killing Floor folder at Documents/My Games/KillingFloor2
3. Delete Killing Floor folder at Steamapps/common/killingfloor2
4. Re-Install Killing Floor 2
Windows 8.1 is probably "unsupported yet" according to their requirements (required/recommended win7 is what they have), but it probably works for most people, so they might be able to help.
I'm actually running the 15.4 beta since the release drivers are getting kind of old and it works fine, but either should work with the game, so I doubt that's the issue.
Downloaded the beta driver. Didn't work. Wiped steam out completely and all games with it. Didn't work. Wiped 4/6 of my hard drives and tried installing on those drives. Didn't work. Tried doing a Windows restore to where I knew it last worked. Didn't work. Ran virus scans on everything. Didn't work. Did a complete wipe including my os drive. Didn't work. Changed out components ( I bought a 970 from best buy knowing I can take it right back and return it. Didn't work. Changed out ram to some of my older ram. Didn't work. Switched to my old quad core phenom 2 setup. Didn't work. I literally have nothing else to try at this point. It was working beautifully and now it just won't.
o man.. you must be raging inside... I hate when things work, then don't. I've even reformatted and done clean windows install, which always works for me.