Could you post a screen shot?  I'm unable to replicate your exact error.  This is what I get depending on the inputs:

Amount blank, Limit blank: "Amount must be specified"
Amount >= 0.001, Limit blank: "Price must be specified"
Amount blank, Limit set properly: "Amount must be specified"
Amount >= 0.001, Limit set properly: order is placed

Amount < 0.001: "Amount must be greater than 0.001"

Amount >= 0.001, Limit set incorrectly: "Post Only: Cannot place order at xxx USD/BTC."

All of these messages appear just below the Place Order button after I press it.

Have you tried reloading the web page?