If you’re trying to use your wordpress.com blog to login to some site via OpenID, you’ll see this error:
Unable to log in with your OpenID provider: Cannot decode Key-Value Form because a line was found without a ':' character. (line 3: '<head>')
[Update] According to the WP support “We have corrected a glitch and the problem should no longer be occurring.”
At least for me, it works again.