Android adb "Unable to open sync connection!"
I was having exactly the same problem, but I already had my phone connected to the computer's USB port. Sometimes disconnecting and reconnecting the cord worked but then it stopped working completely. However, disabling USB debugging on the phone and then re-enabling it has worked so far. Hopefully it keeps working! These fixes really seem like silly hacks.. I'm not sure what the underlying problem is.
I was able to clear this only by cycling the USB Debugging option on the phone.