I never thought I will have such big trouble to buy train tickets online. Some hints for anybody who has not managed it so far:
- Do not even try to register in English, it will fail for some obscure reason.
- Relax your spam filters, otherwise you will not get any mail from them.
- If you have only one credit card, you might be lost. Mine did not work and I had to use another one.
- Be patient, when your third payment attempt ended nowhere.
- If you are not sure whether you booked or not, you can find existing bookings under Mis viajes / Impresión.
But finally I have the tickets from Caceres to Barcelona. I'm going to manage rest on the way.