It's the end of the school holidays in several Northern Federal States (including Berlin) - my guess is that the cheap seats have all been booked up by returning German holidaymakers.

The train booking site for trains in, from and to Germany is www.bahn.de. Travel time is between 6.5 and 7.5 hours depending on connection, and fares for 1 August start at EUR 69 (haven't checked your return date).

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