diff --git a/gaming/troubleshooting.md b/gaming/troubleshooting.md
index d1add29..840b638 100644
--- a/gaming/troubleshooting.md
+++ b/gaming/troubleshooting.md
@@ -214,6 +214,47 @@ For full HDR setup (vk-hdr-layer, KDE config, per-API env vars), see the **steam
 
 **Diagnostic tip**: Look for rapid retry patterns in Pi-hole logs (same domain queried every 1-3s from the Xbox IP) — this signals a blocked domain causing timeout loops.
 
+## Gray Zone Warfare — EAC Failures on Proton (2026-03-31)
+
+**Severity:** High — game unplayable online
+
+**Problem:** EAC errors when connecting to servers on Linux/Proton. Two error codes observed:
+- `0x0002000F` — "The client failed to register in time" (EAC timeout)
+- `0x00020011` — "The client failed to start the session" (EAC session init failure)
+
+Game launches fine, EAC bootstrapper reports success ("Easy Anti-Cheat successfully loaded in-game"), but fails when joining a server at "Synchronizing Live Data".
+
+**Root Cause:** EOS SDK cannot load `steam_api64.dll` at startup:
+```
+LogEOSSDK: Error: Unable to load the Steamworks library: steam_api64.dll
+LogEOSSDK: Error: Integrated Platform Steam failed to get steamworks sdk
+```
+Without Steam integration, EOS has no user identity → `Invalid user EOS ID` → EAC cannot start a client session. The `steam_api64.dll` exists in the game tree at `Engine/Binaries/ThirdParty/Steamworks/Steamv158/Win64/` but EOS isn't resolving that path under Wine/Proton.
+
+Additionally: upgrading from GE-Proton9-25 to GE-Proton10-25 and nuking the compatdata prefix may have triggered the regression. **The game was working earlier the same day on GE-Proton9-25.**
+
+**Log locations:**
+- EAC bootstrapper: `compatdata/2479810/pfx/drive_c/users/steamuser/AppData/Roaming/EasyAntiCheat/.../anticheatlauncher.log`
+- Game log: `compatdata/2479810/pfx/drive_c/users/steamuser/AppData/Local/GZW/Saved/Logs/GZW.log`
+- STL launch log: `~/.config/steamtinkerlaunch/logs/gamelaunch/id/2479810.log`
+
+**What was tried (did NOT fix):**
+1. Installed Proton EasyAntiCheat Runtime (AppID 1826330)
+2. Deleted stale world cache (`GZW/Content/SKALLA/PrebuildWorldData/World/cache/`)
+3. Nuked compatdata prefix for clean rebuild
+4. Verified game files (Steam repaired 20 files)
+5. Upgraded to GE-Proton10-25
+
+**What to try next:**
+1. **Restore GE-Proton9-25** — the version that was working earlier. Old prefix backed up at `compatdata/2479810.bak`
+2. **Check Proton issue [#7721](https://github.com/ValveSoftware/Proton/issues/7721)** — master tracking issue for GZW on Proton
+3. **`split_lock_detect=off`** kernel param — reported to help with EAC init timeouts on some systems
+
+**Lesson:** Don't upgrade Proton versions on a game that's currently working. Pin working Proton versions per-game in STL configs and only upgrade intentionally with a rollback plan (keep the old compatdata prefix).
+
+**Game version:** 0.4.0.0-231948-H (EA Pre-Alpha)
+**STL config:** `~/.config/steamtinkerlaunch/gamecfgs/id/2479810.conf`
+
 ## Useful Commands
 
 ### Check Running Game Process