I also had this problem and just fixed it.
Looks like the new update pushed qBittorrent 4.2.0, which uses a different password hashing process.
You can reset to the default account (admin:adminadmin) by editing the config file at ~/appdata/qbittorrent/qBittorrent/qBittorrent.conf
Delete the lines near the end starting with WebUI\Username and WebUI\Password_ha1
After you log in with the default and set a new username and password, the WebUI\Password_ha1 line becomes WebUI\Password_PBKDF2