After putting around 15 hours into the game, I encountered the "DLC-save-bug" too and I can reproduce it fairly well: The bug always occurs when collecting the Reverse Pulse Cleaner in Vault 3 for the "Still in the Dark" (Broderhood of Steel) quest. I even noticed something else: Without the first patch, all save games crash immediately after I collect the Reverse Pulse Cleaner. With the second patch, I have to leave the area for the save games to fail.
Honestly, i hear people who have not installed the DLC and still have this error message....it is not logical for any programmer to catch a potential exception/error NOT related to installed DLC and still report it as DLC error. This is what bugs me the most...
Think of it this way: Imagine you are selling apples but have a shortage of apples to sell and you are telling your customers: I am sorry but we are short on oranges so go to another apple trader"
Does not make sense? Any fellow programmers around here?
I work as a software developer and this is my thesis: Every item in the game has an attribute which specifies if it belongs to the "main game" or to any kind of DLC. For some items this attribute is not set correctly (i.e. the Reverse Pulse Cleaner). When loading a save state, the application checks if all items in the inventory belong to the main game or to one of the installed DLCs. Since the attribute is messed up for some items, the game "thinks" these items belong to DLCs that are not installed and therefore shows this weird DLC-error-message.