Is 256 a reasonable length for a Diffie-Hellman safe prime?
I'm guessing whoever made the program was confused about key lengths. A 256-bit DH modulus will not give 256-bits of security. It will provide far less.

If you're reading http://www.keylength.com, the value here corresponds to the "discrete logarithm group". These days you really want a minimum of 2048 bits.