I encountered this same error when testing Keycloak 26.1.0 Login using jmeter 5.6.3.

The response data body shows error - Restart login cookie not found. It may have expired; it may have been deleted or cookies are disabled in your browser. If cookies are disabled then enable them. Click Back to Application to login again.

The keycloak logs shows cookie_not_found error - 2025-02-28 16:52:33,459 WARN [org.keycloak.events] (executor-thread-11) type="LOGIN_ERROR", realmId="28bc2e7e-8095-4c80-b05c-da61c242500c", realmName="myrealm", clientId="testclient1", userId="null", ipAddress="127.0.0.1", error="cookie_not_found"

I also updated Realm Setting-> Security Defenses -> content-security-policy to 'self, them'.

Below is my JMeter Setup under Thread Group. 1. HTTP Cookie Manager 2. HTTP Request for www.keycloak.org/app 3. HTTP Request for localhost:8080/realms/myrealm/protocol/openid-connect/auth and set client_id, redirect_uri, state, response_code, response_type, scope, nonce 4. HTTP Request for localhost:8080/realms/myrealm/login-actions/authenticate?session_code=${session_code}&execution=${execution}&client_id=testclient1&tab_id=${tab_id}&client_data=${client_data}

Are there any special settings required? Thanks.