The 2015 error in Discord typically appears when something goes wrong with the cache or the corrupted file or network limitation inhibits the software. The initial step is to clear the cache in Discord close the application entirely and remove the cache folder which is found under:
Windows: AppData > Roaming > discord > Cache
After that, restart Discord.
In case of that, you can also try to reinstall Discord but ensure that you uninstall all previous files before doing so. Check your antivirus or firewall settings, as well, since occasionally the security software blocks the connection with Discord. Finally, make sure your internet is not the problem or VPN/ proxy settings are not the problem, switching it off will usually fix the problem.