Knight Online won't launch on Windows 11 — XignCode3 "Error Code = e0190304", how do I fix it?

I just reinstalled Knight Online from Steam (app 389430) on a fresh Windows 11 machine and I can't get past the anti-cheat. The client starts, the XignCode3 splash pops up, and then it instantly dies with:

A problem occurred in the XignCode3 program.
(Error Code = e0190304)
Terminating program!!!

The game never even reaches the login screen. Here's what I've already tried, none of it worked:

• Ran KnightOnline.exe as Administrator

• Verified integrity of game files through Steam (Properties → Installed Files → Verify)

• Added the whole game folder + XIGNCODE subfolder to my antivirus / Defender exclusions

• Reinstalled the client from scratch

Some old forum threads mention a leftover anti-cheat driver in the registry:

HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\xhunter1

Is deleting that xhunter1 service key and rebooting actually safe, or will it break something? And is e0190304 specifically a driver/registry issue, or is it usually antivirus / a corrupted XIGNCODE folder (read-only attribute, etc.)?

Basically: what's the correct, current fix order for e0190304 on Windows 11, and does the xhunter1 registry step still apply on modern KO builds? Running an RTX card, fully updated GPU drivers, Defender only (no third-party AV).