Hi,

what if you joined new tracker, then you use different client that not LISTED in FAQs section, I think you will got this message
"you are using a banned client"

so I think the best way to know their client first then you may use it

I don`t know that iptorrents gonna say "use qbittorrent"
also most nexus trackers said the same, if you use anything than qbitterrent you will get the above message

but also you can use a client which not recommended by tracker and it will work

for example I seeded my torrents to TLZ using rutorrents, it`s not on their client but it work fine with me

also some Chinese trackers not listed qbittorrent as recommended client but it would work fine

in short, try your client first it may work fine while it`s not recommended by a tracker