When I write my own UIButton-extended class and make it @IBDesignable, I receive two errors in Interface Builder, namely:
I've met the same problem and solved it this way:
error: IB Designables: Failed to update auto layout status: The agent crashed

Then the Xcode will tell you where the prolem is.
In my case I drop the IBDesignable before the class.
Then I clean and rebuild it, the error disappeared