Welcome to the Community,

to find out what can be done, please provide some details:

• what's the vCenter Server version, and build number?
• is it the Windows based version, or the vCenter Server Appliance (a Linux based VM)?
• are you still using the Windows client, or the web client?
• is it the "administrator@vsphere.local" that you need to reset?

You say that the backup doesn't work either. Is the backup configured to use the administrator@vsphere.local account? In this case you may consider to change that to a separate account once the access/login issue is fixed..

André