The title of your post references the control panel. Are you trying to login there?
If you’re still facing an issue, are you using the “Control Panel” button from the client area?
There are two situations I can think of where you might see something like this:
Your account is in the process of being deactivated. In this situation, control panel functionality has already been disabled but the suspension hasn’t fully gone through yet. Within a few minutes, the client area status should update and you will be able to take next steps.
You’re submitting something unusual to the control panel, and you’re tripping a security filter.
If this doesn’t help you, can you please describe what exactly you are doing that is causing you to see this error?