Summary of the problem Minecraft not recieving a non-empy jar file download. EDIT: This is incorrect, I just now figured out that the problem is related to the following line in the crash report. It took me some time to read the report properly, apologies for any confusion. What I had seen was that Minecraft wasn't downloading any of it's assets files, not this error, which I believe is the likely cause of Minecraft not launching: java.lang.RuntimeException: java.lang.RuntimeException: Patcher expecting non-empty class data file for net.minecraft.crash.CrashReport$1, but received empty, your vanilla jar may be corrupt. Pack Version 1.0.0 What is the bug? When Minecraft starts, it tries to download the jar files, but what happens is that it receives an empty jar file download. This means that Minecraft cannot start and therefore exits, dumping control back to the FTB launcher. Please see link to log file for what little detail is available for this startup error. Mod & Version N/A Link to log file http://paste.feed-the-beast.com/view/3ed601ab Is it repeatable? Started Minecraft several times now. Known Fix None known, may possibly be an issue on Mojang's end. I'm filing this report so that FTB can look into it from their end and determine if they've set up anything incorrectly.