Puzzle #1 - Find what is broken - Dealing with tunnels

It seems like everyone likes puzzles, so I thought I would make one.

The diagram below indicates a network where users on router 1 connect to the servers on router 2 via a transport network.

Router 1 and Router 2 had an EIGRP neighbor adjacency using EIGRP and everything was working fine, however, in an effort to move to open standards a similar configuration with OSPF was implemented.  Now, tunnel 0 will not stay up.

The current configurations for R1 and R2 are below.

!!!R1
!
hostname R1
!
!
logging buffered 4096 informational
!
interface Tunnel0
 description ***TUNNEL TO R2***
 ip address 172.18.1.1 255.255.255.252
 tunnel source Loopback0
 tunnel destination 12.43.78.141
!
interface Loopback0
 ip address 12.43.78.129 255.255.255.252
!
interface FastEthernet0/0
 description ***USER NETWORK***
 ip address 172.17.51.1 255.255.255.0
 duplex auto
 speed auto
!
interface Serial0/0
 description ***TRANSPORT LINK1***
 ip address 12.43.78.122 255.255.255.252
 clock rate 2000000
!
!
interface Serial0/1
 description ***TRANSPORT LINK2***
 ip address 12.43.78.126 255.255.255.252
 clock rate 2000000
!
router ospf 1
 log-adjacency-changes
 passive-interface default
 no passive-interface Tunnel0
 network 12.0.0.0 0.255.255.255 area 0
 network 172.16.0.0 0.15.255.255 area 0
!
ip route 12.43.78.140 255.255.255.252 12.43.78.121 name R2_LOOPBACK_NETWORK
ip route 12.43.78.140 255.255.255.252 12.43.78.125 2 name R2_LOOPBACK_NETWORK

!!R2
!
hostname R2
!
!
logging buffered 4096 informational
!
interface Tunnel0
 description ***TUNNEL TO R1***
 ip address 172.18.1.2 255.255.255.252
 tunnel source Loopback0
 tunnel destination 12.43.78.129
!
interface Loopback0
 ip address 12.43.78.141 255.255.255.252
!
interface FastEthernet0/0
 description ***SERVER NETWORK***
 ip address 172.17.100.1 255.255.255.0
 duplex auto
 speed auto
!
interface Serial0/0
 description ***TRANSPORT LINK1***
 ip address 12.43.78.134 255.255.255.252
 clock rate 2000000
!
!
interface Serial0/1
 description ***TRANSPORT LINK2***
 ip address 12.43.78.138 255.255.255.252
 clock rate 2000000
!
router ospf 1
 log-adjacency-changes
 passive-interface default
 no passive-interface Tunnel0
 network 12.0.0.0 0.255.255.255 area 0
 network 172.16.0.0 0.15.255.255 area 0
!
ip route 12.43.78.128 255.255.255.252 12.43.78.133 name R1_LOOPBACK_NETWORK
ip route 12.43.78.128 255.255.255.252 12.43.78.137 2 name R1_LOOPBACK_NETWORK

The EIGRP configuration that was working is below.

!old eigrp section
router eigrp 1
 passive-interface default
 no passive-interface Tunnel0
 network 12.0.0.0
 network 172.16.0.0 255.240.0.0
 no auto-summary

Additionally, the routers can easily ping each other via their loopback addresses that are used for the tunnels.

R1#ping 12.43.78.141 source 12.43.78.129

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.43.78.141, timeout is 2 seconds:
Packet sent with a source address of 12.43.78.129
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/43/76 ms

Why did this tunnel start failing when OSPF was introduced?  What can be done to fix the issue?