I am seeing a similar issue as well. InDesign Server seems to crash after heavy activity, not sure as to the number of documents it opens before it crashes. I get the 3588 error stating to close InDesign documents/books/libraries. I am checking for lock status before I attempt to open any documents, so I am sure its not seeing the file in use. This seems to be a big problem if anyone is planning on using this is an actual production environment. We have thought of running multiple instances of InDesign Server and have our own queuing, but this was prior to reading this post, seems as if I need to look into another solution, or a fix/explanation/workaround from Adobe.