Advice: Check everything over in detail for incorrect paths and typo errors. When I first tried SSO, I had an incorrect file path in the openfire.xml which caused a similar erro as above! Once spotted and fixed SSO worked seamlessly.
Check the path of your gss.conf in your openfire.xml in C:\Program Files\Openfire\conf\
Make sure it is as: C:/Program Files/Openfire/conf.gss.conf