You're missing the timezone in the date format at the end, in your exception message, the "CEST" part.
Your code
DateFormat df = new SimpleDateFormat("EEE, dd MMM yyyy kk:mm:ss", Locale.getDefault());
should be
DateFormat df = new SimpleDateFormat("EEE, dd MMM yyyy kk:mm:ss z", Locale.getDefault());
You might want to read SimpleDateFormat
Edit
At the bottom of this page, the timezone format is more cleary explained
Clearer Timezone format
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