ERROR: Process dir does not contain event

I am afraid I cannot describe my problems well. I am making some tests which are only different in 2 parameters. If I make only a test on the front-end. it will go well. But there are many test files. So I put them on the back-end by &. " Gen_tf.py --ecmEnergy=13000.0 --jobConfig=$PWD --outputEVNTFile=tmp.EVNT.root --maxEvents=10000 &" Then errors come as:

1. /proc/9699: No such file or directory （or no this process ） /proc/9690: No such file or directory..
2. then “ ERROR Process dir /C1N2_Stau_1000_300/PROC_MSSM_SLHA2_0 does not contain events? ” I don't know if, in the back-end, these processes can make a conflict? Since I can run one test well at the front-end.