ValueError: Error downloading data: ProxyError in Python in Excel (Microsoft 365)

"Hello,

I am experiencing a persistent error when trying to retrieve data from the web using the Python in Excel feature (part of Microsoft 365).

The Issue: When executing any Python code within an Excel cell that attempts an HTTP/HTTPS request (e.g., using the requests library), an error occurs. The specific error message is: ValueError: Chyba při stahování dat: HTTPSConnectionPool(host='...') Max retries exceeded with url: / (Caused by ProxyError('Cannot connect to proxy.', OSError('Tunnel connection failed: 400 Bad Request')))

Key Observations and Steps Taken (please review each point):

1. Python in Excel Feature: This issue specifically affects the Python in Excel add-in within Microsoft 365.
2. Consistent Error: The error occurs consistently for any target URL, whether it's https://www.oddsportal.com/ or a simple test page like https://quotes.toscrape.com/ (which is known for easy scraping tests and does not have active protections like Cloudflare).
3. OS Proxy Settings Excluded:
   • All proxy settings in Windows (Automatic proxy setup, Use setup script, Manual proxy setup) are Disabled.
   • The issue is not related to browser proxy settings.
4. Network Connection Excluded:
   • The error persists even when connecting via a mobile hotspot, ruling out the home network (Virgin Media) as the cause.
5. Firewall Excluded:
   • Windows Defender Firewall has been temporarily disabled for all network profiles during testing, and the error still occurs.
   • No other third-party antivirus or firewall software is installed (only Windows Defender).
6. Microsoft Office Reinstallation:
   • Microsoft 365 has been completely reinstalled on the laptop, which did not resolve the issue.
7. Network Reset:
   • A "Network Reset" has been performed in Windows, which reinstalls network adapters and resets network settings. This also did not resolve the issue.
8. System File Check:
   • sfc /scannow was run from an elevated command prompt and found and successfully repaired corrupted system files. However, the ProxyError still persists after a restart.
   • DISM /Online /Cleanup-Image /RestoreHealth was also run.
9. Creative Cloud:
   • It was observed that Creative Cloud performed a self-repair/update after running system file checks, but this did not resolve the network error in Excel.
10. Task Manager Observation:
    • Reviewing Task Manager processes shows no obvious third-party proxy-like processes running that would explain the error.

Conclusion:

Given these comprehensive troubleshooting steps, it appears to be a deeply embedded system-level issue affecting how Python within Excel handles outbound HTTPS requests, manifesting as a ProxyError despite no active proxy being configured. The Tunnel connection failed: 400 Bad Request part of the error suggests an unexpected interception or failure at a low level of the network stack, or within the Python in Excel environment itself.

Any assistance or advanced diagnostic steps would be greatly appreciated."