Image upload in modal window doesn't complete

Hi,
I'm actually not sure, where to post this issue. Ctools could also be a candidate.

When uploading an image via scald form (plupload) in ctools modal, if (and only if!) the file hasn't been uploaded before using the plupload button and users hit the "next" button, upload does not complete.

Uploading starts correctly, one sees the progress bar for a while (screenshot), but then one gets redirected to /atom/add/image/ajax/add and sees:

[{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":...

(It's a hole page full of text, no need to paste everything here, no theme-wrappers, just plain text.)
Of course - the file was not uploaded.

Users have confirmed, that clicking on the "next" button without previously uploading files, has worked before but stopped working weeks ago (somebody else is responsible for updates, ctools was updated around that time).

More system infos:
Drupal 7.51
Ctools 7.x-1.10
Scald 7.x-1.8
Plupload 7.x-1.7
Plupload library 1.5.8
PHP 5.4

Please let me know, if you need additional infos.