[bondant]

Couldn't you book with another ticket seller? Like trainline.eu?

[ivan_gammel]

I tried Omio and one more, but this specific direction was not available there via train. DB website was simply showing an error at the last step. A month later, by the moment error was gone all tickets appeared sold.

[eigenket]

If you run into the problem again try the website of the Polish trains pkp.pl. You'll probably have to Google translate some stuff but it works well.

[octacat]

There is an english version. But pkp site loves to give you errors and timeouts too.