I had ‘same’ error some weeks ago. I added parent of a host, and afterwards it gave the same error if I remember correctly, or something very similar. Took some time to figure out what had happened.
While I was writing a post on this forum (which I cancelled later) of my found ‘bug’, trying to reproduce the issue with screenshots and such, I found out what caused error in my case. Naughty password manager was proactive, autofilling wrong passwords in the SNMP v3 password fields while my focus was on adding parent host info. Fixed it, by putting right passwords back in, AND disabled the autofill feature of my password manager for checkmk server domain. 