Hi ,
We compiled the engine from the 4.12.0 github release branch (we also tried 4.12.1) in order to support XboxOne development. Before that we were using the 4.11.2. I am pretty sure it does not happen in a clean project, and i cannot really tell you how to reproduce the issue.
Within our project we use one persistent level with several sub-levels. By launching in standalon (or packaging), all levels are saved automatically and that’s when the error occurs.
Our workaround (canceling the save for the last two of these sub-levels) lets us package, however, we cannot launch the game in standalone anymore. I could send you our project via pm, if you like.