When I participated in the round yesterday (midnight), after I solved D and had no idea for E, I locked A and tried to make hacks. And then all my Codeforces pages were stuck and kept showing "Just a moment..." "Checking your browser..." "It takes longer time...". I spent the last 15 minutes doing nothing. I felt lucky to submit D before the IP ban. Any ideas?

Yes, I did use vpn. But it's strange when I turned on vpn after the contest ended the IP ban went away. I made the direct connection during the contest.

No, I wasn't sharing wifi with someone else during the contest.