I've read a few times online and got told by a DB employee that you can't buy Spanish seat reservations at the DB anymore so here's a report for any future travellers:
Before you reserve a train via the DB, you need to look up the connection at RENFE's website. If it's not reservable/findable there you can't reserve it via the DB. If it's fully booked the system at the DB will throw a "technical error" and sometimes it could also be thrown without any reason, you just need to try again a few days/weeks later. Newly added train routes and stations are also not instantly available, you need to wait.
At least at a DB Reisezentrum it's still possible to reserve trains. Most employees will look up before how to buy a reservation but one employee was not motivated enough and just said it doesn't work and told me to try it online with the Interrail Booking System. They have an internal knowledge base which has all instructions for buying spanish reservations, e. g. the most important ones for a 2nd class pass: Tarifcode 45, class T for AVE, class 2 for MD/R/RE/Avant and class T or 2 for IC/Alvia but still it doesn't hurt to explain them how it works. A 2nd class reservation should resemble an Elige Estándar ticket.
A note: You should also tell them to search by the train number not by its schedule but the Reisezentrum at Frankfurt Central Station doesn't have the ability to do it so there's a possibility that they can't find a specific train connection which happened only once fortunately. Maybe the Reisezentrum at Frankfurt Süd has it.
Reservations are also 2€ cheaper than on Interrail so it costs either 6.50€ or 10€ depending on the train type. You can also reserve a train (nearly) instantly as it becomes available on the RENFE site, in the Interrail Booking System you can only reserve up to 90 days before which can be too late. I already had a train which was fully booked and needed to find a alternative.
Here are the trains which I've successfully booked, if it helps anyone:
ALV 4186, AVE 3091 (replacement for AVE 3093 which didn't appear in the search), AVE 3310, ALV 190, AVE 2061, AVE 3033
I hope this was helpful.
