After migrating from classic to claim authentication (windows) only some accounts can access SPS 2010

Dear all,

I recently migrated from the classic authentication to claim-based authentication. Some of the users (including the main admin account) now get prompted about the credentials and dont get access to the sharepoint. Other users can freely work in the SharePoint environment. Anyone got an idea what may have happened there? Can I look up, for which users the migration worked?

I already added portalsuperuser and portalsuperreader accounts.

I am using a single server with SP 2010 Server Edition.

Thank you very much!

Best regards,

Benedikt