This question may be related to a resolved thread.
After downloading the site database of the 1.7.1 site and trying two more times to update to 2.4 locally I still get “Site Error: Unable to Load Site Preferences; Invalid Preference Data”. The update process runs through without any error messages, the database is updated, new tables created, the 1.x templates are archived ... I looked at config.php and database.php more than once, can´t find anything.
So I am following Kevins advice and opening a new thread.
Thanks for some advice what I could try.