This is a nasty error, because django-filer just returns this error message in json format when uploading a file via FilerImageField or the djangocms-filer admin interface.
In order to find out the true reason behind this, set the following env vars to true:
- FILER_ENABLE_LOGGING
- FILER_DEBUG
FILER_DEBUG will return a real python exception in the response to the async request that handles the file upload.
In my case, the true error was: decoder jpeg not available which I handled via https://stackoverflow.com/questions/8915296/python-image-library-fails-with-message-decoder-jpeg-not-available-pil
That helped!