When entering values either in "CSS file path" or "Predefined styles path" while leaving "CKEditor default" values in the drop-downs there is no possibility to save the form.

Since settings paths to files is one of the most often issues in CKEditor queue, you should leave the site builder a chance to experiment with default/user styles or their combination which suggests switching between user/default styles. But with the current approach (to not save the form at all) you force us to delete values from those fields. Why do this? This is annoying and doesn't make any sense.

This should not be an error message, instead a warning must be used.