If none of the above steps resolve the issue, you may need to reinstall the Kerio VPN Client:
Conclusion
Error 28201 can be a frustrating issue, but by following these step-by-step troubleshooting guides, you should be able to resolve the problem and establish a secure connection to your Kerio VPN server. If you're still experiencing issues, consider reaching out to your network administrator or Kerio support for further assistance.
Additional Resources
By following these steps and resources, you'll be well on your way to resolving Error 28201 and enjoying a secure and stable VPN connection.
No. It occurs on Windows, macOS, and even Linux versions of the Kerio VPN Client, though Windows is most common.
Outdated clients often break after server updates.
Note: Kerio Control 9.x and newer clients are not backward compatible with very old servers (7.x era). Ensure major version alignment.
| Step | Action | Success Rate |
| :--- | :--- | :--- |
| 1 | Test port 4090 with telnet | High |
| 2 | Disable firewalls/AV temporarily | Medium |
| 3 | Disable SIP ALG on router | Medium |
| 4 | Switch from UDP to TCP | Very High |
| 5 | Re-import .kvp file | Medium |
| 6 | Server license & service check | High (Admin only) |
| 7 | Reset Winsock + disable IPv6 | Low |
When none of the above work, dive into the logs. Open the latest .log file (e.g., KerioVPNClient_20250115.log) and look for lines containing 28201.
Example snippet:
[ERROR] [VPN] SSL handshake failed: certificate verify failed (error: 28201)
[INFO] Server certificate common name does not match requested hostname.
This confirms a hostname mismatch. Another common line:
[ERROR] [VPN] Connection aborted: unsupported protocol version (28201)
That points to outdated client or server TLS version mismatch (e.g., server requires TLS 1.2 but client tries 1.0).
Use the log to pinpoint the exact reason.