Hi Guys,
To me this looks like a cipher issue.
There are a few options that you can try to resolve this if it is.
I would try adding the following lines into your STunnel Configuration file:
delay = yes
options = NO_SSLv2
options = NO_SSLv3
options = NO_TLSv1
options = CIPHER_SERVER_PREFERENCE
options = DONT_INSERT_EMPTY_FRAGMENTS
Delay will delay and DNS lookups that maybe actioned by the request (not normally needed but I always include if for sanity sake)