I’m not a SAML expert but it sounds like signature verification failed…
No really, without at least some more details such as your saml provider config (remove secrets of course), it will be pretty much impossible to help you.
A link to your Indico server or the SAML metadata (server on /multipass/saml/<yourprovidername>) would also be helpful.
You can also try setting "debug": true in the SAML config to get some more verbose output (ideally you do this on a dev instance where you can easily see such output)