I have seen this before and believe this
translates to the OSPF
neighbor changing state from FULL to EXSTART because of the receipt of a
Database Description (DBD) packet from the neighbor with an unexpected sequence
SeqNumberMismatch means that a DBD packet during OSPF neighborship negotiation has been received that either:
This can be attributed to (Unidirectional) packet loss as a possible cause.
So I would take a look at the port that the neighbour is connecting through and check for any packet loss.