Agent exited with code 255: Host key verification failed

Troubleshooting
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2

UPDATE: WORKS!

What I did:
su cmk ssh -i checkmk_ssh root@<target-server>

Asked me to trust the fingerprint. YES. Now it works!

Edit: Here the full command for anybody with same problem:
ssh -o StrictHostKeyChecking=no -i $OMD_ROOT/ssh/checkmk_ssh -T root@$_HOSTADDRESS_4$

Thanks @aeckstein