For Proxmox VE, the default login is User ID root, and the Installation time Password, for some reason, If the root password is forgotten, or not working then it can be reset using the following procedure:
Boot the Proxmox VE in Grub Edit mode by pressing e on the keyboard, this is shown in the screenshot.
In the Grub-edit mode scroll down to the highlighted line, and add the line: init=/bin/bash in the following manner as shown in the screenshots.
After adding the line press Ctrl x, this will boot the system in a Linux bash shell.
Next, type the following command:
mount -rw -o remount /
This will mount the root directory in read/write mode.
After this, type the new password using the following syntax. In case of multiple users, you may replace “root” with another user name.
passwd root “Enter New Password”
Next, enter the password again to confirm.
Lastly, manually reboot your Proxmox VE Server, this will reset the default login password.