Discord is one of those applications that is a PITA (Pain In The Acetabulum). The problem is that updating Discord is not a matter of simply running sudo apt update && sudo apt upgrade, the app likes to do its own thing. When Discord has an update, it starts the download of a new .deb (Debian Package) when launched. Discord won’t load if there’s an update waiting that hasn’t been applied.
Unfortunately, if you just try to open the Discord Debian package using the “software” application, it will not update correctly (unlike most applications).
My solution is simple: remove discord (but don’t use the purge command), and install the Debian package manually using dpkg -i. To remove the old package:
sudo apt remove discord
This removes Discord, but rarely removes settings. So if you’ve already set your credentials for Discord you won’t likely have to set them again. (At least most of the time). To install Discord, change to the directory you downloaded discord to, likely downloads, then run dpkg -i on the discord package name:
sudo dpkg -i discord-0.0.25.deb
In this case, I ran ls to list the contents of the directory. The new version of Discord was discord-0.0.25.deb. And voilà, Discord works again.