Password-login-forbidden

Can a native speaker please comment here? I am not sure what is the correct grammatical form.

Siebrand13:38, 29 December 2010

I have consulted with a friend of mine who is an English teacher and she confirms that it should be 'this' because username and password are both in the singular. The message is however a little ambiguous - should this really be 'or' instead of 'and'? If it is 'or' then it would be better style to write 'the use of this username or this password has been forbidden'.

Lloffiwr21:37, 1 January 2011