Error, Unknown (All versions) will not work at [privatetrackername] please use another client (Not private site friendly)
Is this a problem with qBittorrent or could it be an issue specific to the private tracker?
I get the same results on both Windows and Linux, and on the same Linux computer I am able to successfully download from the same private tracker using Transmission instead.
The site in question probably uses either a blacklist or whitelist to control what client you use, most likely they also have a list of acceptable clients, ask them if or why it's not allowed and if they would allow it, which they should. Or else it's time to find a new private tracker.