IPSec Tunnel from OLD OS to new OS

[jparadise]

We just installed a new 140D at one of our branch locations which has the newest OS on it.

With every other branch site, we have easily set up Site-to-Site IPSec tunnels (including the one in question on its older FG) back to the main office (300D).

However, going through the exact same setup (using the wizard), we are unable to bring the tunnel up.

Is there a chance that the 140D's new OS version is not playing well with the existing version on the 300D (maybe 2 versions behind)?

[ede_pfau]

Which versions are we talking about (branch new/branch old/HQ)?

I'd compare the config files from the old and the new release of the branch FGT. There shouldn't be many changes in IPsec VPN across, say, v5.2.1 and v5.2.5.