Came across an interesting issue with an ESXi box recently - the root credentials were working when accessing the server via iKVM, however when trying to login from vSphere - an error was thrown stating an incorrect username or password was being entered - when that obviously wasn't the case.
To overcome this issue follow the steps below:
1) SSH into the ESXi box with the root details
2) Try resetting the password for the user and set them as Admin:
esxcli system set --id root -p (enter existing password)
esxcli system permission set -id root --role Admin
(Try logging in via vSphere using root - if unsuccessful continue the steps below)
3) Create a new ESXi user with the following command
esxcli system account add –id sauadmin –p 12345 –c 12345
(obviously changing the passwords so you don't use the ones listed here.)
4) esxcli system permission set --id sauadmin --role Admin
5) Log into vSphere using the new admin user - make changes as required - and reboot if required.