What do the logs say?. You might need to bump up the log level (global Administration > System > Log Level)
Make sure you upgrade to SonarQube v9.9 LTS soon, not only to benefit from our Best LTS Ever™, but because soon we will systematically ask users to upgrade when they ask questions about earlier versions of SonarQube, which are now considered unsupported.
SonarQube 9.9 LTS also offers a dedicated space in the UI for debugging SAML integration. I highly encourage you to upgrade.
It looks like the SAML response is received at a different address than expected. What are you using to serve SonarQube over HTTPS (IIS, Nginx, Apache…)