That save string is 6169 characters long. All Base64-encoded strings have to be a multiple of 4 characters long, so the closest it should be is either 6168 or 6172 characters. I tried writing a quick test which runs through the entire string, removing each character and testing if that makes it valid, but no luck there. So either you accidentally deleted/didn't copy 3 or more characters, or at least 5 characters were accidentally added.
FWIW, the discord link is set to never expire, and still works for me. I have had people have issues with it before though. I believe if you open discord, click the + button at the bottom of the server list, then "Join a Server" at the bottom and paste the link it, then it should work. Among other things, the discord has a save bank. It doesn't cover everything, but if you don't have any other saves backed up, it should get you close to where you were.