This happens when you add a new Super User and you manually deactivated Admin Tools. This means that Admin Tools has no records about you creating the Super User, so it thinks it's a malicious user.

Before continuing, let's solve the root cause of this issue. Most likely you got a 403 error creating a new Super User and as temporary solution you manually disabled Admin Tools. Please check this page of the troubleshooter to find the correct solution for this problem.

Then enable Super User monitor feature and get inside the profile of the user that was disabled. Save it without changing anything. In this way Admin Tools should record the new Super User being a legit one. Try again to login with the user that was previously blocked; you should be able to use it, now.