I see. Thus the problem is that IPWorksOpenPGP does not create proper OpenPGP private keys. I guess they use OpenSSL with their different CRT parameter style and do not convert them correctly. RFC-4880 says this in 5.5.3:
The secret key is this series of multiprecision integers: o MPI of RSA secret exponent d; o MPI of RSA secret prime value p; o MPI of RSA secret prime value q (p < q); o MPI of u, the multiplicative inverse of p, mod q.