Reproduction steps:
1. Open the attached “IN-113695_ProxyBug - Latest.zip“ project (link in the internal comments)
2. Open the “SampleScene” (make sure it is added in the Build Profiles Scene List)
3. Go to Windows Settings > Network & internet > Proxy and make sure “Automatically detect settings” is set to “Off”
4. In Windows Proxy Settings > Use setup script, press the “Set up” button
5. Turn “On” the “Use setup script” option and for the “Script address” field enter “http://127.0.0.1/wpad_doubleproxy.dat”
6. Open the project’s “Assets” folder in the Windows File Explorer
7. Right-mouse click inside the “Assets” folder, select “Open in Terminal”
8. In the terminal, enter the “python3 -m http.server 80“ command (Python 3 must be installed on the computer)
9. Build and run the opened Unity project
10. Wait for the project to open and then a few more seconds afterwards
11. Inspect the Player.log file

Actual result: Unsupported proxy syntax error (“Curl error 5”)
Expected result: Request timeout error (“Curl error 28”) or a successful request

Reproducible with: 2022.1.0a6, 6000.0.67f1, 6000.3.8f1, 6000.4.0b8, 6000.5.0a6
Not reproducible with: 2022.1.0a5

Reproduced on: Windows 11 Pro (25H2)
Not reproducible with: No other environment tested

Error messages:
”Curl error 5: Unsupported proxy syntax in '<…>;<…>': Port number was not a decimal number between 0 and 65535
Request failed!”
”Curl error 28: Failed to connect to <…> port 80 after 21045 ms: Could not connect to server
Request failed!”

Notes:
- To get the expected result, replace the “Script address” in the Windows Proxy Settings with “http://127.0.0.1/wpad_singleproxy.dat”
- Reproduction in Play mode is inconsistent, so it is best to try reproducing in the Player only
- Might not reproduce on the first try, but should reproduce at least from the second try