clEnqueueTask() failed with error code CL_INVALID_COMMAND_QUEUE.

Looks like you are trying to modify this code:

https://github.com/fpga-opencl-benchmarks/rodinia_fpga/blob/master/opencl/backprop/backprop_ocl.cpp

I guess you did not create the cmd_queue2 queue using clCreateCommandQueue() and that is why you are getting that error. If you attach your complete host code, I can give you a more concrete answer.