Sounds like a problem with the host. If the data you placed in th econfig file is correct and you are still getting errors then the host must have changed something.
Did you get the guestbook from here or did your host provide it for you? I hate it when hosts provide scripts as they mangle them and make help harder. Integrating the guestbook into a site is simple but hard to explain. There are threads dedicated to it. Basically you just take all the HTML that will appear before the guestbook and put it in the header.pgp file found in templates, remembering to keep the CSS and body tag in place. Then copy all the HTML that will appear after the guestbook into footer.php.
I have now made a page where I can list all the modifications I have made for Advanced Guestbook. The mod files are txt files and should be simple enough for everybody to follow. the page is www.carbonize.co.uk/AG/
If there is a particular mod you would like then contact me either using the forums (this one or preferably my own) or by email.
BTW you know you could integrate the guestbook into your site so that you don't have to have it open in a new page. Have a look at the guestbook on my site to see what I mean.