Try publishing a constant string each time, rather then creating a new string each time. That would remove the possibility of the sprintf() function causing an issue (implementations can do unexpected things), and potentially the buffer being access from more than one thread simultaneously (just looking for something that could cause a data corruption). Some something like:
static const char * const StringToSend = "Send me to the clouds";
So its not on the stack either.
If that doesn’t move you forward please attach your FreeRTOSConfig.h file here (or send it to me a r dot barry at freertos dot org if you don’t want to attach).