I have a setup where a bunch of remote site are interconnected via a provider's MPLS network. I connect to the at the various remote sites using BGP on a combination of serial (T1's) and TLS links. Rather than to redistribute my routes via BGP I elected to build my own OSPF across GRE tunnels. The problem is that some OSPF sessions come up ok but now they seem not to. I get stuck in EXCHANGE/EXSTART mode. I am getting the initial DBD and it is being ack'd but the subsequent DBDs are seen by one of the remote routers and ACK'd but the ACK never makes it back to the originating router. I'm sure it is MTU related somehow but I can't tell where the problem is. Both ends of the tunnel have an MTU of 1524. One end is sourced from an ethernet interfae while the remote end is sourced from a serial link. The largest non fragmented ping I can do across the tunnel is 1500. The size of the DBD is 1452. I would think that should be able to come across but it isn't.
I've attached some debugs and config snips.