I hadn't played in about 11 months (since Jan 2012), which was the majority of 2012, and two expansion packs were released, neither of which I grabbed. (I have Realm of the Diggle Gods, as I did when I started this game.) Somewhere along the way (Steam cloud or otherwise), the save or the game has become... well... interesting. This save seems corrupt but playable, in that items I carry, either equipped on in my belt, along with shop items and spell skills, are duplicated and spread out along the level, usually on walkable terrain but sometimes in walls, over water or lava, or straight up invisible. These items all think they're the same item as the original that they duplicate. Sending all the artifacts in a stack of artifacts I have to the museum causes a crash the next time I move. Artifacts that are shop items can't be sent to the museum ("what are you doing with that?"). If I pick up an item that's a shop item and walk into a store, I can't sell it. If I interact with other items I'll need to drop it before Brax will let me walk out (if I don't interact with any items, then he won't have moved, and I'm a thief). I can pick up a shop item and offer it to Krong and then it's a different item. Spell on the ground: http://i.imgur.com/SNv3v.png Picking up a spell and turning it into lutefisk causes a crash. Picking up a spell and walking while holding it causes a crash. Picking up a spell and dropping it (as I tried to do to access an item underneath it) causes a crash. I think I also had some crash with carrying duplicate wands but I don't remember; I just avoid them now. I'll also note, as the screenshot shows, that despite not having the proper expansion, I have access to the Pocket Dimension, though the cool stuff the wiki lists (device in the north through which I can enter keywords, device in the center through which I can change the decorations) are missing.
Everyone can access the Wizardlands, regardless of whether they own Conquest. Steam, eh? Well, try these things first: 1. Start a new save with a different name (if your last save on your above character is 'Automatic Save', then save properly on that character beforehand) and see what happens. It might take an hour or so to see if the symptoms are still there. 2. Verify the game files. Right click it in your library, go to properties, then cycle through the tabs until you find the "Verify Integrity of Game Cache" button. Click it, let it work (Steam's main window will become unresponsive whilst this happens, I believe) and if anything is not correct then the game will fix your files. I, personally, also recommend not using Steam Cloud unless you absolutely have to. I find it causes more issues than it's worth.
Made a new character and let it sit for a while before continuing, as you suggested (though a few days instead of an hour). Didn't manage to reproduce the issue, though.