What I did immediately before this happened:
I run my torrent client on my NAS. I wanted to test out downloading a torrent with a VPN enabled on the NAS.
It worked very well and it downloaded 2 torrents that I added.
After removing those torrents from the client I disconnected the VPN.
Since then I have not been able to download any new torrents and the existing torrents in my client are no longer seeding.

What I have done to troubleshoot:
I exited the torrent client and shut it down completely
I logged out of GT and logged back in
I opened the torrent client again and tried adding a new torrent
I tried downloading a torrent from another tracker - with no success
Finally, I enabled the VPN again and I was able to add new torrents and they downloaded fine and I could seed fine

Error messages:
In tracker information qbittorrent is giving me the message "Host not found (non-authoritative), try again later"

I haven't changed any network settings – all I did was switch a VPN on and then off. And now I can't download anything without the VPN on.
It's obviously not a GT issue, it's either something local on my network or something going on with my ISP (less likely).
Any help or suggestions greatly appreciated!