This error was reported back in March 2018 for qbittorrent-nox: [HowTo] qBittorrent on OSMC - #46 by amnesia89 The version in Debian stretch is 3.3.7, which is rather old (latest seems to be 4.2.1). The version in Debian buster is 4.1.5, so perhaps the problem will have been fixed by then.
In your qbittorrent service file, you could choose to send all error messages to /dev/null, instead of the system journal. It will, of course, send all error messages, which might bring its own problems.
In the [Service] section you’d need to add the line:
StandardError=null