Admin block listing showing unwanted debug info
That looks like someone put a print_r or similar debugging call into your copy of Seven. If you are using git (which your project should be) you will be able to see the change in that tool and undo it.  The other solution would be to download a fresh copy of Drupal and replace the files in the core directory with the ones in the fresh copy.

If there are multiple people working on the project, ask around about who was testing out ideas and ask them to undo the changes to the admin theme.