Under what conditions does it do it? I noticed that when I first open a project from a version earlier than 15.2 the bottom left corner of the screen will display this message:
I was not able to capture the messages before it said 'all media loaded' but I have seen this process take a few min and the media was pending while it was doing it. It seems, that based on your disk speed and volume of media, this can take a few minutes or more.
If I let it finish, it works fine, and contained to to be fine the next time I load the project. I have not seen this before the 15.2 update, so keep that in mind.