You can't reset the web admin password from the customer facing cli, you can change it but you need to know the old password.
Sometimes, this issue is caused because the user is using IE Explorer to access the web GUI. You can try using Firefox or Chrome or click on the compatibility view via IE Explorer and then try to log in again.
Outside of the above option, a support contract on the DXi v1000 would be needed so Quantum could access the DXi with root privileges and manually reset the web admin password.