Unfortunately this is a known issue with the current version of the Excel Plugin. Because this issue affects a low number of users and the Excel plugin is currently undergoing a full rewrite, the decision was made not to fix the defect. Engineering resources will instead be used to work on the new version of the plugin.
While we await the new version of the Excel plugin to be released, users can request to be put on the SSO Exceptions list by their local Subscription Administrators. This will allow the user to login to Agile Central (including the Excel Plugin) using their Username and Password. Users will no longer receive the parsing error and will be able to connect using the Excel plugin. This is the only known workaround for users experiencing this error.