If you have a local user with a password that contains the mentioned characters, after the upgrade, you will not be able to sign into the GUI or SSH with that user.
corrupts local user passwords with special characters !#[.]\=
Upgrade corrupts password
There are three workarounds to this issue:
Create a new user prior to upgrade, and refrain from using the mentioned characters within the password. After upgrading, sign in with the newly created user, and set the old user password with the same password, then logout, and login with the old user. Afterwards, you may delete the user created prior to upgrade.
Change the user password prior to upgrade. Be sure to omit the above mentioned characters. Upgrade, then revert the password to the previous after completion.
Upgrade without changing the password. Use the serial console, and initiate the initial configuration wizard to set your password to what you had prior to upgrade.