Thanks for the link! In the end, I got someone to send me the Unofficial legacy discord and was able to solve my issue there. To save a click, you just need to replace the authsrv.crt in the UnionClient haven.jar. Open the jar with 7zip/winrar and go into the haven folder and replace the current authsrv.crt with the one from
here. I had to extract the current authsrv.crt and edit it with notepad, then add the file back.