The account running demonbuddy joins someone elses game. (Manually using the quick join over friendlist)
When the other person leaves the game this pops up. (Every time, at least in short games where no quest progress was made - like your usual sub 1 min farm run).
Also, pressing OK while in the next game often makes you leave that game or crash Diablo as well.
(This seems to only occour with DB running, without it on vanilla Diablo it doesnt seem to occour)
To prevent it, it would need to happen right when it pops up or during the short time while waiting for the next game to be created.