I guess I just found the worst #booking system possible: the spanish #renfe. After doing all the countless steps to book a two-way trip with #AVE (select trains, select options, enter all passenger info even including document IDs, select payment method), it stops with an uncomprehensible error message and all you can do is "accept" you can't book this 🤯
Finally found the cause of the error: there were no seats left on the train back in the category I tried to book. I learned this by trying to book at some reseller, THEIR system could tell me.
So, went back to renfe and booked there (to avoid the extra provision of the reseller), selecting the "better" ("first class"?) category for the return, which finally worked fine.
I'm still happy with what I got: less than 50€ in total for a two-way trip, distance almost 150km, in a modern high-speed train taking just an hour, for an adult and a child ... that's an awesome price compared to e.g. germany. But their booking system is seriously from hell.