There is an error if you try to login to an IdP with the Http-POST binding.
The POST request will have a 'SAMLResponse' parameter instead of a 'SAMLRequest' parameter.
When i found this, i first believed that the error was in the HTTPPostEncoder class of OpenSAML library.
I update it to the latest version, (modifying the MetadataMemoryProvider to implements new method, correct the initialization process). It doesn't correct the problem.
After searching a little more, I found out that the HTTPPostEncoder used in the HTTPPostBinding class is from the wrong package (saml1 instead of saml2)...
I attach the patch to correct this.
This patch also includes the code to upgrade openSAML version to 2.4.1