Why can't I undelegate my SP to @minnowbooster?

Hello, steemians!

Sometimes ago I delegated ~67 SP to @minnowbooster.

Back then I just wanted to try their service. There weren't any problems to do it. However right now I'm trying to cancel my delegating to them, but I simply can't.

1. So, firstly I go to FAQ and use the link for undelegation procedure
   

2. On the next webpage I press CONTINUE button
   

3. Then I fill in my username on steemit.com which is @everythink, and fill in my active key. Next I press SIGN IN button.
   

4. And after that I get following error message:
   

How should I solve it??

Note: My active key I take from here:

PS: Using main password instead of active key doesn't solve the problem either.