HEF2011: As far as I can remember, without the official patch, the script trigger that ends certain missions in M.A.X.2 will not work properly.
The beauty of having M.A.X.2 patched is that the player can play every single mission in the campaign without success; If the player fails a mission, M.A.X.2 will not hold the player back. Campaign missions can be skipped and the player can move onto the next one.
Rodor: Do you have the patched version? Patching GOG version of M.A..X.2 with official Interplay patch 1.40 makes game campaigns unplayable for me: it says "Wrong save file - can't load". Though scenarios run fine.
I own the retail version of M.A.X.2 and yes, it is patched at v1.40; My patch came from either a older PC GAMER cd, or a dedicated online site. Either way, I kept the patch stored on a disc for years.
I have never had any problems installing or launching M.A.X.2 and I've owned various computers over the years since the day the game came out without a issue. Ever.
Try using a different save slot in the Load screen.
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It wouldn't hurt to uninstall M.A.X.2 and complete a fresh re-install, would it?