At this time,  Your Library 
 has difficulty performing operations on more than ~30 items 
simultaneously. It should, however, be capable of performing as expected
 with a few items at a time. We're sorry for the inconvenience.  Please let us know if the issues you are experiencing involve batch-deleting a  smaller  number of assets at one time. If that is the case, we'll look more into troubleshooting.  Also, please note that  the Roll20 Dev Team is working to improve the Art Library as we speak :    Steve K.  said:  Art
 library management is something that's important to the Roll20 Dev 
team. It's something we consider a priority, however the API 
infrastructure has to come first, which will be the focus of our next 
update. I'm hoping the marketplace and art library management comes 
shortly after that.