The OS I tried are XP x86 SP3 and XP x64 SP2, both with the latest updates.
Step to reproduce this:
1) from my local Firefox installation I open a magnet link in "qBittorrentPortable.exe".
2) qBittorrent opens and adds the link.
3) from my browser I try to open another magnet link in "qBittorrentPortable.exe", while qBittorrent is already running.
Result: Firefox can't resolve the address, claiming the magnet protocol isn't installed or has some problems. I also tried with Chrome and it says nothing, but still doesn't open the second magnet if qBittorrent is already running.
Note that I have manually associated the magnet protocol to "qBittorrentPortable.exe" in the windows registry.
I haven't ticked the option "use qBittorrent for magnet links" inside the qBittorrent settings, because that makes the application not portable anymore: it would make me open the magnet link directly in "qBittorrent.exe", skipping the "qBittorrentPortable.exe" launcher.
Am I missing something?