Here's Let's Encrypt's post about the expiration:
https://letsencrypt.org/docs/dst-root-c ... mber-2021/
Both the client system and server system needs to trust the "ISRG Root X1" certificate. Here is a partial list of systems that trust "ISRG Root X1":
- Windows >= XP SP3 (assuming Automatic Root Certificate Update isn’t manually disabled)
- macOS >= 10.12.1
- iOS >= 10 (iOS 9 does not include it)
- iPhone 5 and above can upgrade to iOS 10 and can thus trust ISRG Root X1
- Android >= 7.1.1 (but Android >= 2.3.6 will work by default due to Let's Encrypt's special cross-sign)
- Mozilla Firefox >= 50.0
- Ubuntu >= xenial / 16.04 (with updates applied)
- Debian >= jessie / 8 (with updates applied)
- Java 8 >= 8u141
- Java 7 >= 7u151
- NSS >= 3.26
A player on Windows 7 was unable to access the server list in the game (specifically getting a "Can't talk with list server" message), but after installing Microsoft's KB3004394 update, it worked again.
My Debian 9 server didn't have all its updates installed and none of the servers it was hosting were showing up on the list. After installing updates, which included some for curl and openssl, and restarting my bzfs servers, they were back on the list.