Yes, that could very well be the cause of your problem. Each save game contains references to all event files used by the scenario when the scenario was started. Note: it contains file name references, not the actual file contents. When loading a save game, those references are followed, and the game tries to reload those event files.
If for some reason one or more of those files is no longer present, you end up with the error you received. Now, the AI has been altered significantly in both version 1.1 and 1.2, and has resulted in a different set of AI files.
What you should do, is to start a new scenario with your current version of HOI 2, save the game, and reload that save. If the error is no longer there, then your problem is indeed what I described above. Then you basically have three choices.
1) Abandon the old game, as it has started under an older version of the game. This might be a good idea anyway, as you won't benefit from other improvements either, such as tech teams, leaders, ministers, etc.
2) Downgrade (aka: reinstall from scratch) the original 1.0 version and do not patch, if you really want to continue your current save game.
3) Manually edit out the references to the missing event files in your save game. I would not recommend it, as you basically destroy part of what the AI makes into an, well, AI, as defined by the scenario you originally started.
I would suggest you follow 1)
