I'm having a similar problem, with 4 seats in C, travelling MIA-FRA next week.

I selected seats several months back, but now getting the "not possible/cancelled" message. There appear to be no alternative seats available.

Check My Trip (but not miles and more) shows that we've been reallocated to different seats, sitting separately, elsewhere on the plane. Two of the party are 12 year old children, so this isn't ideal.

Availability is showing as 0 in all booking classes except 2 seats in each of Y and B, so obviously a pretty full flight.

A couple of calls to LH's US reservations line have proved unhelpful. In both cases, the advice is there's nothing I can do other than try to change seats at checkin at the airport.

Is there anything else I could try? I've no status with LH. I appreciate the situation isn't desperate here, but frustrating not to be able to sit with my children. On what might be an oversold flight, is there any possibility at all of a voluntary re-route? Final destination is MAN, and could travel anything up to +3 days.