[Bug] Arduino IDE 2.x hangs on startup (gRPC ETIMEDOUT 127.0.0.1) on Windows 11 - FIXED ONLY by Windows 8 Compatibility Mode

Development Tools IDE 2.x
1

Arduino IDE 2.x gets completely stuck on the startup screen (splash screen). The logs show that the frontend (Theia/Electron) fails to connect to the backend daemon (arduino-cli) via the local loopback address (localhost), resulting in a persistent Error: 14 UNAVAILABLE: No connection established. Last error: Error: connect ETIMEDOUT 127.0.0.1:<port> error.

After hours of troubleshooting, I discovered that running the Arduino IDE executable in "Windows 8 Compatibility Mode" instantly fixes the issue.

To Reproduce It's an environment-specific issue on Windows 11, likely related to how the OS network stack interacts with the Electron framework when specific virtual network adapters are present in the system.

  1. Launch Arduino IDE normally on Windows 11.
  2. The IDE hangs indefinitely on the splash screen.
  3. Check logs: The arduino-cli daemon starts successfully and listens on a random port (e.g., 127.0.0.1:63899), but the frontend times out trying to connect to it.

Troubleshooting steps taken (None of these worked):

  • Downgrading versions: Tested on IDE 2.3.8, 2.3.3, and 2.3.2. Same issue on all.
  • Reinstalling: Installed for "Current User" and "All Users".
  • Antivirus/Firewall: Completely disabled ESET Antivirus and Firewall.
  • Clearing Caches/Configs: Deleted everything inside:
    • C:\Users\<user>\.arduinoIDE
    • C:\Users\<user>\AppData\Local\Arduino15
    • C:\Users\<user>\AppData\Roaming\arduino-ide
  • Network Resets: Performed netsh winsock reset and rebooted.
  • VPNs: I have ZeroTier and Cloudflare WARP installed, but both were completely disabled/turned off during testing.

The Workaround (What actually worked):

  1. Right-click on the Arduino IDE shortcut or Arduino IDE.exe.
  2. Go to Properties -> Compatibility tab.
  3. Check "Run this program in compatibility mode for: Windows 8".
  4. Apply and launch. The IDE starts instantly and works flawlessly.

The IDE should be able to resolve and connect to its internal localhost process on a native Windows 11 environment without requiring legacy compatibility modes.

Desktop Environment:

  • OS: Windows 11
  • IDE Version: 2.3.8 (and older 2.3.x)
  • Security: ESET (disabled during test)
  • Network: ZeroTier / Cloudflare WARP installed (but disabled)

Logs Snippet:

Plaintext2026-05-10 14:15:51 2026-05-10T12:15:51.092Z daemon INFO Daemon is now listening on 127.0.0.1:63899 {"IP":"127.0.0.1","Port":"63899"} 2026-05-10 14:15:51 2026-05-10T12:15:51.092Z daemon INFO Daemon is running. (...) 2026-05-10 14:15:51 2026-05-10T12:15:51.422Z root ERROR Uncaught Exception: Error: 14 UNAVAILABLE: No connection established. Last error: Error: connect ETIMEDOUT 127.0.0.1:63899 2026-05-10 14:15:51 2026-05-10T12:15:51.422Z root ERROR Error: 14 UNAVAILABLE: No connection established. Last error: Error: connect ETIMEDOUT 127.0.0.1:63899 at t.callErrorFromStatus (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:3120831)

It seems that the presence of virtual network adapters (ZeroTier/WARP) in Windows 11 breaks something in how Node.js/Electron handles IPv4 loopback (127.0.0.1) vs IPv6 (::1), even when those adapters are turned off. Forcing Windows 8 compatibility likely changes the networking API calls the app uses under the hood, bypassing the issue.

2

Good report. If I see anyone having that initial error we can now try the Win 8 trick. My only concern is does that 'break' something else less obvious. I think the chances are low but must be kept in mind. Hopefully you sent all the information to Windows and the VPN companies.

3

Interesting workaround.
Since Windows 8 compatibility mode fixes it immediately, this may be related to the Windows/network environment rather than the Arduino installation itself.

Good to know!

4

My money is on the AV and VPN. Did you try safe mode without the Win 8 compat? I think that completely eliminates the AV/VPN issue.

5

Fortunately (or unfortunately) I can't reproduce your issue on my Win11 system.