Steam Client Browser Error Code -138 - Unknown error

[Goolean]

Since 2 days the Steam client web browser will only show the Error Code 138 - Failed to load webpage (unknown error). This applies to store, community, 3rd party websites etc. In addition, friendlist is unable to connect (friends network unreachable).
Web browser in big picture mode functions correctly, the News snippets on the library window also load correctly.

Additional Info:
No changes in internet connection. First occurrence after shutting down the PC over night. Win7

Steps I tried to resolve the issue without success:
- Firewall switched off
- Port forwarding
- clean Steam reinstall
- switch to beta branch
- disable/enable DirectWrite option
----------- Edits -------------------
- Delete Web Browser Cache: no result, both from client and deleting the files manually
- DNS server set to Google: no result




Any ideas appreciated.

[.357]

Steam > Settings > Web Browser > Delete Web Browser Cache.

Firstly try that and restart the client.

[Goolean]

Issue persists after:
Steam > Settings > Web Browser > Delete Web Browser Cache

[.357]

Now, change your DNS server settings.

Use GoogleDNS or OpenDNS.

Once that is done, reboot your PC.

https://developers.google.com/speed/public-dns/docs/using

See if anything has changed.

[Goolean]

DNS change to google didn't solve the issue. How would this affect the Steam browser alone?

[.357]

Through CEF [Chromium Embedded Framework} (which the Steam client uses), the error code 138 typically means Network Access Denied.

This is used to distinguish errors that were most likely caused by a firewall from other access denied errors.