Just adding to this, I had this same issue and eventually had issues logging into the server even through direct connection using copy/paste passwords from a password manager. I eventually enabled the SSH to the server and suddenly the web interface on the ESXi server worked and I could add to vSphere using the same clipboard copy of the password.
Using vSphere Client version 6.7.0.42000 and VMware ESXi, 6.7.0, 14320388
Hope this helps someone else.