The SCALD modal window ‘loading’ wasn’t loading. It took a couple of days (hours) and an embarassing long discussion on drupal.org to find the error – which was not an error. It was ‘just’ a problem of re-declaring something, that was already declared before.

The short version:
“The WYSIWYG module was detected. Using both modules at the same time may cause problems. It is recommended to turn the WYSIWYG module off (click here to disable).”
[reading (and understanding) a warning on the CKEditor profile page can actually be useful!]

The long version: …to be continued…

The documentation of the bug hunt: https://www.drupal.org/node/2454845