Yup.. it was the Bettercream for sure. You can't use whipped icings under fondant. Like Vicki said, the moisture makes your fondant turn into a gooey mess. If you want an icing with the texture of bettercream, that holds up under, and won't ruin your fondant try a meringue buttercream like SMBC.
I have to tell you all that I often use filling with wipped cream under fondant, and also as a crumb coat. The trick is to use a very good quality fondant and to let the whipped filling 'breathe' in the fridge for at least 4 hours before covering. I don't know about MMF since I have never tried this.
In fact any kind of filling with cream, cream cheese or mascarpone has to breathe so as not to make the fondant crumble or 'sweat'.
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I use bettercream as filling all the time with no problem.. Is using it for covering a cake with fondant when it starts to create problems. But as an icng by itself is good. All you have to know is that is a softer icing and it wont crust.