A network engineer is troubleshooting OSPFv3 for IPv6 between two directly connected routers, R1 and R2, on a point-to-point link. The neighbor relationship is stuck in the EXSTART/EXCHANGE state and never reaches FULL. Interface configurations show matching areas, matching timers, and both interfaces are up. Running 'debug ipv6 ospf adj' reveals repeated DBD packet retransmissions. What is the MOST likely cause of this problem?