It could be that in the original 7.01 forum sho ever set up the forum disabled the password encryption. Have a look in the tblAuthor table in the database, if the Password column is plain text passwords then it is disabled.
Another thing it may be is the page encoding. This is the encoding used by the browser when submitting a form or displaying content. Back with 7.01 this was iso-8859-1 but is now utf-8 for improve language support.
Try editing in notepad the file includes/setup_options_inc.asp and changing the page encoding from utf-8 to iso-8859-1 and see if that allows you to login.