Very strange... now I can get back into Dun without a problem...
Well, here's what happened up to the Dun crashing issue... I had reinstalled AutoBot (as I hadn't used it in a long time), and I wonder if its installer did something with my \data folder. I deleted the \data folder in the Mabinogi folder and then copied over my backup \data folder, and could get back into Dun again.
Now for the testing portion... I reinstalled AutoBot once again, and again entering Dun would immediately close the client. Unpacked and renamed the \db folder to disable it (the usual culprit for client crashes) and I could enter Dun again. Renamed it back and then renamed propdb.xml... and that was the culprit. Seems that AutoBot's updater overwrote my propdb.xml with an old one. It was missing a LOT of props, as well as changed variables for existing props.