CIPSEND to ESP8266 returning bytes but no content
In my experience, the "busy" message appears when you try to start a new AT command or to send data before the ESP8266 is ready. You say that you are running this test with the commands in "a script" which suggests to me that there is no significant delay between lines.

You should either wait between commands until the EXP8266 responds (usually with "OK" or ">") or add a delay of a few seconds between commands.