The $ symbol is reserved to access cvars, that causes trouble here. The Login button just executes
profile_login $menu_profile_file $menu_profile_pass
where menu_profile_file and menu_profile_pass are cvars you can set above by selecting the profile and typing in a password. The variable names will just be replaced with what these vars contain before executing the command. If your variable contains any reserved special chars (I think the dollar sign ($) and quotation mark (") are the only reserved ASCII chars), this will cause trouble because the game thinks you want to use these with their special meaning altough you want them as plaintext.
For example:
menu_profile_file MyAccount
menu_profile_pass $MyPass
profile_login $menu_profile_file $menu_profile_pass
will evaluate to
profile_login MyAccount $MyPass
$MyPass is not set here since you never intended it to be a variable. You can go around that by typing in your password in quotation marks ("). This would result in:
menu_profile_file MyAccount
menu_profile_pass "$MyPass"
profile_login $menu_profile_file $menu_profile_pass
will evaluate to
profile_login MyAccount "$MyPass"
Here, the game parses $MyPass as plaintext and does not give an error.