Your LimeSurvey version: 6.4.0+231218 
Own server or LimeSurvey hosting: own server IIS 10 + PHP 8.1.22.
Survey theme/template: Bootswatch
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I have installed this third party plugin github.com/BDSU/limesurvey-oauth2 after modifying the config.xml file in the zip file to add version 6.
Authentication is working (I get a token and LimeSurvey creates a session), but I get stuck into infinite loops between /index.php?r=admin/authentication/sa/login which redirects to OAuth server which redirects to  survey-uat.cbre.fr/index.php?r=admin%2fa...n%2fsa%2flogin&code= ... which redirects to /index.php?r=admin which redirect to /index.php?r=admin/authentication/sa/login which redirect to OAuth server and so on...

I don't know what's causing the issue since the plugin code looks OK compared to native LDAP and Webserver plugins. If I block the redirection after newUserSession successful execution, I can dump the cookies and session variables and everything looks OK. I can even navigate manually to /index.php?r=surveyAdministration/listsurveys and I'm authenticated.

Do you have any idea about what could cause the issue?

I'm posting this here since the plugin does not seems actively developed and the issue may not be the plugin since the OAuth authentication is working.

By the way, it would be great to have native SAML and OAuth2in Limesurvey.

Really strange …

Seems you are connected …

Workaround : at github.com/BDSU/limesurvey-oauth2/blob/5.../AuthOAuth2.php#L201

Check if already connected ?