Hello ramis
That might also be because there has popped up some planned engineering works or something which will mean that the train is going to be diverted and therefore it is not possible to reserve seats, or maybe the vehicle will be a different one than ususal. In any case, there not being any more reservable spaces does not at all mean that there will be no seats. I would be quite surprised if you don’t find a seat, that only happens really rarely. It is possible tough that you will not find a free seating group of four seats, so if you are travelling together with others you might in the worst case be separated.