Welcome to the Community,
from what you explain this could be an issue with Security Group Policies.
Some accounts need to have e.g. "Logon Batch" permissions, and if a Group Policy removes these accounts you'll end up with a system that works for the time until the policies are applied again. Youll find the information in the documentation at VMware vSphere 6.5 Documentation Library
Note that the required information seems to be only available in the English version of the documentation!
From C:\program files\VMware\vcenter server\bin run the service-control --status --all command and verify the services are running.
I had a similar issue. The accounts were not set to do not expire and were checked to have the user change the password at next login. I had to reset the local user accounts used by the vCenter services. I then had to update the account passwords in vCenter, i.e. "c:\program files\VMware\vCenter Server\vmon\vmon - p eam" then enter the new password. I believe there was about 6 or so accounts I had to set.
Thanks, this worked for me