The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
2026.7.1 is not a test version of the client and no changes to connection logic were made compared to 2026.7.0.
2026.7.1 should be able to play with anyone. Most likely it was just antivirus or some other network issue.