Cutting and pasting the "Sign client SSL certificate" from NuSphere Technical FAQ yielded the result:

139703278187840:error:02001002:system library:fopen:No such file or directory:../crypto/bio/bss_file.c:69:fopen('-out','r')
139703278187840:error:2006D080:BIO routines:BIO_new_file:no such file:../crypto/bio/bss_file.c:76:

The command from the FAQ reads:
openssl ca -verbose -config openssl-ca.cnf -policy signing_policy -extensions signing_req -infiles clientcert.csr -out clientcert.pem

Just change the -infiles clientcert.csr to -in clientcert.csr and it will work.

Hi EK, thanks for posting this
I think our failt is we posted -out at the end to follow similar pattern for all the commands. We should've kept it before -infiles because -infiles should be the last argument