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No, I didn't touch the router. Both Deluge and QBit 3.0.8 worked fine, so it is hard to see how the router would only affect 3.0.9.

Deluge also uses libTorrent I think? Maybe it re-set something on my computer? I have no idea to say the least!

In any case, it works now. Thanks for your time.

I think I have found out what is happening. The torrent site I usually use has a maximum of 50 active torrents. 3.0.9 seems to be reporting the wrong number of active torrents. The trackers will not allow the download to start if the site shows over 50 active torrents.

I just had it not start a new torrent again. I reverted to 3.0.8, and noticed the torrent site was reporting 51 active torrents. When I forced it to re-check the number with 3.0.8, it dropped to 31.

I'll have to go back to 3.0.9 and see if I can duplicate this, and what the tracker status says when it is stalled.

Edit: 3.0.9 seems to report the total number of active torrents to the site, rather than the number only from that site. I have torrents from another site as well, and when the total goes over 50, new torrents just stay stalled. It seems if I force the site to display the active torrents, then the number drops to 31 and everything is fine. 3.0.8 doesn't seem to do this.

Downloaded this and qbit started downloading files again. 

Whilst downloading from Mega I also had to check TLS usage as ok.  Don't know whether this is related to stalling problem.