I have randomly noticed the blocked seatmap problem as well over the last couple of weeks. You open the seatmap on LH.com and it appears that all seats are taken.
This can happen in any class of booking, no matter how long out the flight is.
Sometimes it only affects a single segment in the booking.
However, your travel agent can reserve your seats regardless over the GDS and so can LH call centers. It is just a problem that self-service doesn't work.
I am not sure what causes this error, but it is definitely an IT error.