Shoreditch theme breaks CKeditor Link dialog box
I just tested with Shoreditch activated as CiviCRM custom css, with seven as admin theme and CK Editor worked just fine.

I also tried to replicate your setup, using Drupal 7, Shoreditch 0.1-alpha25, Civi 5.5.3, Garland as CiviCRM admin theme, and custom CSS URL as other instead of shoreditch and I was unable to replicate your issue. 

It seems that you are not actually using shoreditch as your civicrm theme. What is listed as your theme when you look at Administer - System Settings - Resource URLs - Custom CSS URL? Do you typically use Garland as an Admin theme?  I found that the most compatible theme with shoreditch (when used as Custom CSS) is Seven, as I ran into trouble with other admin themes such as adminimal.