"target server failed to respond" only when using HTTPS?

I’m using soapUI (v4.5.2) for load testing of a web services application server (based on Axis2+Tomcat).
By sending request to test one same web service method, using Simple Strategy:

- When the endpoint is set to use HTTPS, it is stable at 50 TPS. But if increase the thread number to a higher value (above 60), the load test will report error of:
“org.apache.http.NoHttpResponseException: The target server failed to respond”
While there is no exception happened on the web services application. The CPU/memory usage is not very high.

- When the endpoint is set to use HTTP, then there is no error at all on the load test results (even works fine at 100 TPS).

So, it seems the bottleneck is not on the application server itself, but about HTTPS? but why? and how to improve?


Thanks a lot!

p.s.
Tomcat https connector configuration:
maxThreads="200" minSpareThreads="80" maxSpareThreads="80"
enableLookups="false" disableUploadTimeout="true"
acceptCount="100"
scheme="https" secure="true" clientAuth="false" sslProtocol="TLS"