DH Key agreement parameter validation failure

[Joseph Galbraith]

Hello,

I have a set of DH key agreement parameters that
are being negotiated as part of a protocol.
When talking to a different server (not our implementation),
the parameters do not validate.
  CryptoPP::DL_GroupParameters<CryptoPP::Integer>::Validate()

calls

  ValidateElement(level,
                  GetSubgroupGenerator(),
                  &GetBasePrecomputation())
which checks that

  Jacobi(g, p) == 1.

This check is what is failing.

So, I want to be able to talk intelligently
(or at least some reasonable facsimile thereof)
to the the implementer of the server about what
the problem is with the DH parameters they are
giving me.
So, my question is, what is the meaning of
this failure?  What are the security implications?
Thanks for any answers,

Joseph