Not quite sure what you mean. Do you get asked for the password when you are trying to log on to the web UI or is it when you thy to open folders in network neighbourhood?
If the former log on to the unit and check what it is showing in the admin logon page (passwords are case sensitive)
If the latter log on to the unit and check the user account settings (again passwords are case sensitive)
If the settings in the unit are correct then it is a windows problem most probably an update has changed the password case sensitivity in which case you need to change that.
I tend to suspect windows networking is the problem.
I think OP has had "mapped network drive" and I have a similar issue but I know and remember the password.
On booting to desktop the device is there but needs password to connect - if I try to use "remember password" it doesn't remember so each boot to desktop I will have to enter password.
With OP I believe he had the same but selected remember password and for 3 years they have had no issue - their OS has remembered the password and it auto connects on reboot.
Now I'm not sure if this will work but can OP access web gui by typing IP address in browser and if so do they access via administrator level or by user account? and if user account only... can they change the default admin level password?
HTH and hope you understand