Source code button: (Chrome): Cannot read property 'config' of undefined (Firefox): TypeError: a is undefined

When i click on "Source", Ckeditor switches to the source code, which is good.
But when i click on "Source" again i get the message:

Chrome:
ckeditor.js:786 Uncaught TypeError: Cannot read property 'config' of undefined

Firefox:
ckeditor.js:786 TypeError: a is undefined

I use CKEditor 4, i even downloaded the newest one, removed all other javascripts from my page except jQuery and CKEditor but still the same error.

This is my CKEditor config file:

CKEDITOR.replace( 'editor1', {
    extraPlugins: 'autogrow,codemirror',
    removePlugins: 'resize'
});

CKEDITOR.on('instanceReady', function(ev) { ev.editor.fire('contentDom'); });

Thanks for your help.