My friends and I are planning on taking the overnight train from Lisbon to Madrid in late June. All the websites I've tried to book through though come up with errors - either saying the tickets are not available, that there's a problem and I need to contact the train company itself, and the online chat help said that the tickets were all sold out for that train - but the Renfe website itself says they're fine! Plus it's still a long way out so I don't see how they could be sold out now. Does anyone know what is going on? I'm scared that if I wait too long I will miss out on tickets and we only have one possible date to travel on! What should I do next? Please help!

There's a timetable change* around Mid-June and bookings after it aren't opened yet. They should be already opened but, as usual, they are late. Stay tunned to see when bookings are opened for that route (my guess would be that they'll be opened in early June but it's just a guess. It could be in May or in Mid-June...).
*It's unlikely the schedule for that specific train will change. It's just that there are two general schedules along the year: one valid from Mid-December till around Mid-June and another one valid from around Mid-June to around Mid-December.