ESXi 6.7: Suddenly unable to login

I am currently setting up a new Server with ESXi 6.7 standalone (DELL PowerEdge R620, not part of a vSphere Cluster).
This is day two for me working on the Server and for the third time I am suddenly unable to login via HTTP or SSH:

The message for both is simply wrong username/password.
I always have to login directly on the Server’s ESXi Backend (which is working fine) and do Troubleshooting Options > Restart Management Agents for it to work again. This is a fresh install of ESXi from the 6.7 Image, all I’ve done so far is installing updates like this:

esxcli network firewall ruleset set -e true -r httpClient
esxcli software profile update -p ESXi-6.7.0-20180604001-standard -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml
esxcli network firewall ruleset set -e false -r httpClient
# Reboot to complete the upgrade

I already had this problem a couple of weeks ago on another Server (also ESXi 6.7), but the problem only occured once after changing the root password and since it never occured again and was workin just fine after restarting the Management Agents once I didn’t bother about it.

I did not find any helpful advice searching for this problem online. Any help/advice would be greatly appreaciated as I would really like to do something about this before leaving the country.
This is the current ESXi version of the server:

[root@sbeesx:~] esxcli system version get
   Product: VMware ESXi
   Version: 6.7.0
   Build: Releasebuild-8941472
   Update: 0
   Patch: 14

EDIT: Just happened again, added the poper screenshots.
EDIT2: Just had the phenomenon an an ESXi host managed by a vCenter Server