I know that sometimes this message comes when it should not appear. I filed ticket 9567 for it.
The case is: when you've modeled a BeforeDelete event (and this microflow returns true), the object gets deleted AND this message appears.
In the supportal I saw it is scheduled to be solved in 2.5.8.
If your have modeled such scenario's this might be the problem
It means that the object you want the retrieve no longer exists. This can be either because you never commited it and lost it from your context. Or because you deleted the object somewhere in your microflow.