There are a few historical conversations about this error on Github that I would recommend looking through: React LoginCallback Component: "AuthSdkError: Could not load PKCE codeVerifier from storage. This may indicate the auth flow has already completed or multiple auth flows are executing concurrently. · Issue #898 · okta/okta-oidc-js · GitHub
Are you only seeing this error occur when the user attempts to come back to the application after their token’s have expired? Is autoRenew set to true (this is the default value) for OktaAuth?