This error is caused by a problem with a custom attribute plug-in.  The "My custom error message" is the text of an exception thrown by the custom class so will differ between systems, however the AttributePluginException is the indicator as to where to look. 

The error must also be view in the context of where the error has been generated, in this specific example the error has occurred at the Relying Party (RP) after the SAML response has been sent by the Asserting Party (AP).  By enabling the display of SAML messages (see How to display SAML protocol messages in RSA Federated Identity Manager (FIM)).

In this instance the only fix available is to debug the custom code, the FIM 2.6 server is running correctly and has passed attributes from a SAML response into the custom plugin.

For more details on creation and implementation of FIM plugin see the associated documentation, also available in SecurCare Online

RSA Federated Identity Manager 2.6 Planning and Installation Guide

https://knowledge.rsasecurity.com/docs/rsa_fim/fim26/PlanInstall.pdf

RSA Federated Identity Manager 2.6 Developer's Documentation

https://knowledge.rsasecurity.com/docs/rsa_fim/fim26/devguide/docs/index.htm