a search showed up, that the same key is in App_GlobalResources\SharedResources.resx file contained as well, I assume, that the key in login is no longer used.

Jason,

yes, please log this as a task into the public issue tracker at support.dotnetnuke.com. Thank you.

Sebastian,

I have found that the value of the above mentioned resource is in fact used, at least in one situation: When the Admin of the site forces a password reset and the user must change his password on his next login. If he enters a wrongly configured new password, then this message is the one that shows up.

The message under analysis is the following:
'You must enter a valid password. Please check with the Portal Administrator if you do not know the password requirements.'

Which is stored inside:
Admin - Authentication - App_LocalResources - Login.ascx.resx
Resource Name: PasswordInvalid.Text

IMHO it may be useful that there exist different messages for different situations, since a forced password reset may lead the user to re-enter his old kind of password and then a specific message, mentioning something special regarding this situation may be of help.

So I would consider keeping both messages. The only problem I found is that in this second message (within Login.ascx.resx), if I try to include the tokens [PasswordLength] and  [NoneAlphabet], they are not properly parsed.

Saludos,
Alejandro.