Uhm, thanks.

I can't say that I know much about dealing with MAF errors. 1 MB sounds like a Huge map,18 civs, in the late game. Were you still able to load the save back in the day, or had you stopped playing because it kept crashing? I'm wondering if using the specific version you had at the time would help, but perhaps you've already tried that. Trying the latest BtS version was also a good idea, I think. I'm not sure if savegames have remained backwards-compatible all the way through from version 1.0 of the basegame to 3.19 of BtS, but I guess Firaxis have at least tried never to break savegames. And it sounds like you're getting the same error message with Steam BtS as with your older version (just the basegame, I assume); I'd expect it to crash earlier and without a proper error message if compatibility were the problem. Well, it couldn't hurt to zip the savegame and upload it as an attachment; unless that's somehow too personal. Possibly, removing some of the game's art assets could reduce the memory use enough to at least get it to load once.
If you search, e.g.
site:civfanatics.com civ 4 "maf"
on Google, there's a lot of discussion on the forums about working around those errors, a lot of things to try, most of them probably futile.
Edit: Re-reading your post, I suppose it doesn't necessarily crash already while loading; maybe only when moving the camera or ending the turn. In that case, removing some, say, less important cities and units through WorldBuilder might be an option. And you're not sure whether you've tried it with BtS. Well, I've run a quick test – no, BtS won't just load a basegame save. I should've expected that too; BtS made a lot of XML changes, and with Civ 4's non-annotated, stream-of-bytes savegame format, it would've been a big programming effort to maintain compatibility with the basegame. I could still try loading your save with my copy of the basegame.