The variables $samlNameIdNameQualifier and $samlNameIdSPNameQualifier are defined at the top of the SLO login and never used within the saml_slo function.

Values ARE assigned to $nameIdNameQualifier and $nameIdSPNameQualifier but not previously defined.

https://github.com/onelogin/wordpress-saml/blob/e0e1e68dea4ba100f380efcd7c2331d379376867/onelogin-saml-sso/php/functions.php#L178-L205

I suspect these variables are intended to be the same. With WP_DEBUG on this breaks the SLO process with undefined variable warnings sent before headers are sent to the browser.