I'm not sure what was the real problem but I've:
1. reinstalled uTorrent
2. changed the port uTorrent was using in the router
3. disabled Windows firewall and then enabled it
4. set my PC's IP as DMZ in the router
5. 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.