Today I copied and pasted my VMware ESXi root password to the vSphere client password box and hit enter to login. When I looked back at notepad I saw I didn’t copy all the characters of the password. So the password I pasted into the vSphere client was not correct.
After some research (Google… 🙂 ) I found out this is a known issue on ESXi 4.1. If you have a password with >8 characters only the first 8 will be checked during login. So if you password is for example vmware1234567 you can successfully login with the password vmware12 or vmware1209876.
A workaround for this issue is described at VMware KB artikel 1025400, a permanent fix is not yet released.