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root@mylinuxhost~# ss -tulpn | grep 6556  

root@mylinuxhost~# ps waux | grep cmk-agent-ctl
 disable the login to this account

root@mylinuxhost~# cmk-agent-ctl status  

root@mylinuxhost~# systemctl status check-mk-agent.socket

root@mylinuxhost~# systemctl status cmk-agent-ctl-daemon.service

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• It is obvious, but it needs to be said: Check whether the username and the password or the secret are correct.
• This error message can also be displayed when the option disable the login to this account is activated for this user. So please go to Setup > Users > Users in Checkmk and check if you see the following icon in the row for the user you are trying to authenticate with: 
  In this case, you have to edit the user and deactivate the checkbox in front of disable the login to this account.

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