The binary package qbittorrent 3.3.7-1 in the archive has been built against libtorrent-rasterbar-dev 1.1.0. Rebuilding qbittorrent against 1.1.1 seems to fix the issue.
OTOH, qbittorrent built against 1.1.1 but run with libtorrent-rasterbar9 1.1.0-3 will crash upon adding a torrent with the following message: qbittorrent: symbol lookup error: qbittorrent: undefined symbol: _ ZN10libtorrent8bitfield6resizeEi It looks like the two versions are ABI incompatible and a soname bump was required.
