I have GRE tunnels in multiple places, manually setup over an IPSEC tunnel. It works great, except at 1 place I have this oddity:

I have an IPSec tunnel, 10.0.0.1 at site-A and 10.0.0.2 at site-B.

The routers can ping each other with those addresses, no problem there, the IPSec tunnel is solid.

Now, I setup a GRE tunnel:
Site-A
Local-Address = 10.0.0.1
Remote-Address = 10.0.0.2

Site-B
Local-Address = 10.0.0.2
Remote-Address = 10.0.0.1

That config generally works everywhere except for this place.

Here, at this place, the only way I can get the GRE tunnel to work is if I remove Site-B's Local-Address, like so:

Site-A
Local-Address = 10.0.0.1
Remote-Address = 10.0.0.2

Site-B
Local-Address = leave blank
Remote-Address = 10.0.0.1

The GRE tunnel now works perfectly, and everything is up and running. As soon as I put in a local-address there, GRE goes down again.

I'm glad it is working, but what I can't figure out, is WHY this happens. I have compared IPSec and GRE configurations at this place and another place where the local-address works, and I simply can't find any differences.

Does anyone have any ideas about why putting the local-address into the GRE tunnel makes the GRE tunnel stop working?

The local-address has to be a valid address on the router, and when there is NAT involved it has to be the NAT'ted address (not the external address).

The local-address has to be a valid address on the router, and when there is NAT involved it has to be the NAT'ted address (not the external address).

Yes, it is. It is attached to an empty bridge. And, I can ping both addresses from both routers.

I cannot confirm your problem, it works OK for me.
You probably have a minor mistake or typo that you overlook by checking it again and again.