Hello
I'm not sure why you are having this specific problem.
I had problems trying to set the cipher to AES128-SHA. I solved the problem by installing the newest version of stunnel. (4.18) which has OpenSSL 0.9.8. Now it works with no problems. (It did not work with openssl 0.9.7)
You can also try using the "sslVersion" option (see manual) to set it to TLSv1 or SSLv3 see if that makes a difference.
Good luck