Stash developer here. That doesn't look good. We usually see problems like this due to proxy configuration. What are you using to provide the https - just Tomcat, or something like Apache?
You can try enabling debugging in Git/Curl to get more information:
export GIT_TRACE=1
export GIT_TRACE_PACKET=1
export GIT_CURL_VERBOSE=1
git clone --debug https://admin@stash-url/path/to/repository.git
In any case I would contacting Atlassian support, attaching the output from that command, so we can help you resolve the problem.
Cheers, Charles