I’m not sure what was the real problem but I’ve:
- reinstalled uTorrent
- changed the port uTorrent was using in the router
- disabled Windows firewall and then enabled it
- set my PC’s IP as DMZ in the router
- set-up port forwarding again.
I’m not sure which step solved the problem but now everything works like it used to.
Thanks for your support and for suggesting port-related problems.