This is likely from attempting to serve a non-image ad (Custom, HTML5, Text, etc) into a Zone with size restrictions (a Zone with values set on the Min Width, Max Width, Min Height and Max Height fields). AdPlugg uses those fields to ensure that a mis-sized Ad doesn't end up in the Zone - messing up your layout. Unfortunately, the settings only work with traditional image ads at this point. With other ad formats (such as Custom, HTML5, Text, etc), when the system can't determine what the size of the Ad is, it will err on the side of caution and keep the Ad out of the Zone (because it doesn't know whether or not the Ad meets the size requirements). To fix, clear the values out of the Min Width, Max Width, Min Height and Max Height fields for any Zones where you want non-Image ads to be able to appear.
Sorry for the trouble. We are considering revising how this works as it has caused difficulty for other users in the past. Please contact us with any questions, suggestions, etc.