I found the reason: I had polish character in comment in /etc/apt/sources.list. Just sth like:
# Wyłączone w trakcie upgrade do saucy deb .... (after I removed this comment, the error went away, now I get „can't calculate upgrade” but that's another story) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1309447 Title: Unicode decode error during upgrade to 14.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1309447/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
