Staff received massive bug reports of qB 4.2.0 recently.
After some test, it shows qB 4.2.0, which used libtorrent-rasterbar 1.2, suffers from severe low connectivity.
It's bugs include not being able to corrently report ipv6 address, or blindly rejecting all connections in certain conditions, etc.
Hence we banned qB 4.2.0.

Here are suggestions:
For users currently on qB 4.1.9 or lower, keep where you are.
For users currently on qB 4.2.0, we recommend downgrading to qB 4.1.9. If you are seeding a large volume of torrents and thus can not do re-hashing for this downgrade, do an upgrade to qB 4.2.1 (a bugfix version) instead.