Tensorflow: terminate called after throwing an instance of 'std::system_error'

My script fails always on session = tf.Session(config=config) with this error

terminate called after throwing an instance of 'std::system_error' what(): Resource temporarily unavailable Neúspěšně ukončen (SIGABRT)

import os
import subprocess
import tensorflow as tf

def setGPU():
    free_gpu = subprocess.check_output(
        'nvidia-smi -q | grep "Minor\|Processes" | grep "None" -B1 | tr -d " " | cut -d ":" -f2 | sed -n "1p"', shell=True)

    if len(free_gpu) == 0:
        print ('No free GPU available!')
        sys.exit(1)

    os.environ['CUDA_VISIBLE_DEVICES'] = free_gpu.decode().strip()
    print('I got GPU!')

    return int(free_gpu.strip())

setGPU()
config = tf.ConfigProto()
config.gpu_options.per_process_gpu_memory_fraction = 0.5
session = tf.Session(config=config)

Did anybody encounter something similer - any advice?