Problem description: after growing the topology of the network, some OSPFv3 neigbhbors were stuck in the Loading phase
 
Solution: the size of the LSA update didn't fit anymore in the IPv6 MTU of the link and the router (Brocade MLXE Ironware 5.2f and 5.4c) didn't handle the case correctly. Increasing the IPv6 MTU of the link fixed the issue.