From Tunngle Wiki
Error 4-104: Could not reach the Tunngle master server.
- A Firewall could be blocking the communication.
- On some rare occasions the master servers could be down because of the ordinary maintenance work (that is normally announced on the Tunngle website and the community forum).
- You are using an old version. Versions below 18.104.22.168 are not supported anymore.
- Check that TnglCrtl.exe (you can find it in the Tunngle main folder) is not blocked by any Firewall. We strongly recommend that you add a permanent firewall rule for this executable.
- When troubleshooting please note that certain firewalls could interfere with Tunngle even when they appear to be disabled since some security settings might be managed outside the application rule list.
- If the servers are undergoing maintenance (as announced on the website and on the forum) all you have to do is wait until the work is done. The connectivity will be restored as soon as the work is over. It won’t take long: we do our best to reduce the service outages to the minimum possible. Keep in mind that the maintenance of the Tunngle servers is normally announced on both the Tunngle Website and the Community forum.
- If you are using an obsolete version you can try to:
- Download again from here