I'm completely new to SSO, so please bare with me... I am trying to setup Salesforce as the IDP using SAML 2.0 for a third party application called Aha.io. I've followed this guide to Enable Salesforce as an Identity Provider:
https://help.salesforce.com/apex/HTViewHelpDoc?id=identity_provider_enable.htm&language=en (https://help.salesforce.com/apex/HTViewHelpDoc?id=identity_provider_enable.htm&language=en)

After setting up my domain and enabling the required items, I then went to my Identity Provider Setup, and downloaded the Metadata XML file, which I then uploaded into Aha in their SAML 2.0 Configuration Section (they have an option to upload a Metadata file to be read for setup).

Now at this point, when I try to go to our aha.io portal (http://msidev.ideas.aha.io), it attempts to redirect me to Salesforce for my Single Sign-On, but instead of being logged in and redirected back to Aha, I receive the error "Invalid HTTP Method".

At this point, I'm stuck. I can't find any documentation specific to this Single Sign-On error, or any additional setup instructions for setting up SFDC as the IDP. Has anyone encountered this error before? I don't know if there is a problem with my IDP setup, or if there is a problem with Salesforce attempting to redirect me back to the service provider after authentication. The Identity Provider Event Log is blank, so I have no information to go off of.

Can I assume that the metadata that I used to setup SFDC as my IDP is correct and that Aha is correctly bringing me to Salesforce to authenticate? If so, why am I getting this error instead of being passed back to Aha?

I'll take any help I can get, please. Let's solve this together!