When I try to temp ban offline players, it broadcasts in chat but the player doesn't actually get banned, they can still join the server. However if they are online it works perfectly.

I checked the logs for errors and there wasn't any.

Can you paste the server log please when this happens. Also, has the player ever logged in before?

Yes the player has joined before.

The issue has seamed to have stopped now, and I can't find when it happened in the logs.

But now there is a new line when the player is temped ban which was not there before in the logs;

[19:50:13] [Server thread/WARN]: [AutoSaveWorld] Plugin Essentials attempted to establish connection socket://api.mojang.com:443 in main server thread

Which was not there when the error was occurring from what I re-call.

Closing due to inactivity; if this is still a problem, please open a new issue.