Several weaknesses have been discovered in how Diffie-Hellman key exchange is being deployed in many servers and clients. Good thing to know is that with the default settings, the Hiawatha webserver is not vulnerable. Make sure you didn't set the DHsize setting to a lower value than its default value of 2048. For future releases, I will make 2048 the minimum value.
You can use this webpage to test whether your server is vulnerable or not.