modal window "loading" not loading in SCALD
Fixed it: the problem was that after going through the SCALD installation process the CKEditor was activated and the plugins declared in two locations: 


under WYSIWYG Profiles
under CKEditor Profiles


This also corresponds with the error message I had in FF console:

uncaught exception: The editor instance "edit-body-und-0-value" is already attached to the provided element.


I simply deleted the WYSIWYG Profile and the problem was gone – modal window loading as it should. The detailled documentation of the process finding the error can be found over here: https://www.drupal.org/node/2454845#comment-9749453