qB 4.4.0 introduced BT v2 protocol, and you can choose when you make torrent:

1. v1 mode (current mode)
2. v1 mode + file alignment
3. hybrid mode (default for qB 4.4.0)
4. v2 mode

Where qB 4.4.0 supports all
qB < 4.4.0 and most clients support 1/2/3
uT 1.x/2.x supports 1, and has bad support for 2/3, which has some compatibility problem.

We know tracker still has a certain % of uT 1.x/2.x access, so uploading of 2/3/4 mode torrents were currently not allowed.
Please use mode 1 when you make torrents.

We suggest users who is still using uT 1.x/2.x consider upgrade to uT 3.x or switch to other clients. Staff may add support for mode 2/3 and drop support for uT 1.x/2.x in future.

Uploading v2 mode torrent will not be allowed in foreseeable future.