I looks like something on your network is breaking the connection between your device and your computer. The fact that you must power-cycle the device implies that the problem is somewhere inside your kindle, because it is holding on to the network socket. I suspect that waiting a couple of minutes for the network to time out will also work. It could also be something related to your wireless router and interference from neighbor access points.
Debug logs might help determine what is happening. Follow the instructions in
this FAQ answer. Once you have calibre running in debug mode, try sending the book. When it fails, quit calibre, capture the debug log and email it to me at cc_debug_logs at charles dot haleys dot org. When you send the CC debug log, please erase the MultiPie address. No reason to clutter my son's inbox.