Yes, that's the reason.
You're user 2.
User 1 offered two of his games.
Then he removed at least one from his tradables.
Then you accepted the offer. At this point, Lestrade's doesn't know what it 'should' see as the accepted games, making it possible that the trade will fail. In these situations you should contact user 1 by PM, ask them to re-add their games if they still have them and edit their offer, or cancel their offer.
BVG's way of doing this is 'simpler', it remembers games rather than tradables, meaning it's easier for it to 'recover' from such an error but also makes it possible for the offer to fail because for instance user 1 has an extra copy as a Steam gift and won't trade that copy for your game. See what I mean..?
I didn't get any feedback on my way of doing it, so I assumed it was best to do it that way. I could, of course, simply disable the Accept Offer button if an error is found in the trade offer like here... Or, alternatively, scan through their current tradables, try to find a newer tradable with the same characteristics to replace the old tradable, but that also means having to remember the store ID from the original offer... And... Well, meh..?