I apologize to Stormgarde, not meaning to state the obvious things when troubleshooting mods. :thumbsup:
No apology necessary - your suggestions were appreciated although I think in this case I don't think it's one of the more obvious reasons. Your suggestion to disable Immersive Interiors got me thinking about trying to disable another mod that actually uses scripting. Immersive Interiors doesn't do anything with scripts, even though it is a big mod it doesn't really matter as far as the OBSE co-save is going to be concerned. However, disabling a "big" random mod with scripts - well, that makes a difference. And that does reinforce my thought about some kind of resource limit being hit in OSR when loading a game. It seems other people have had similar issues yet with different mod setups.