This could mean that the installation directory path you used while installing Elgg was incorrect. Can you access your database? If so, look in the elgg_datalists table for an entry where name = path. Make sure this is the correct path to your Elgg installation on the server. It needs to have a trailing slash at the end.
If you can't or are uncomfortable accessing the database, try deleting everything and starting over, but checking that the installation path is correct when you install.
Quick word of thanks to Brett. After moving my site to a new server I had the same problem as the OP. I found this discussion and tried checking the elgg_datalists table. Bingo! I changed the paths to the correct directory and everything started working again.
info@elgg.org
Security issues should be reported to security@elgg.org!
©2014 the Elgg Foundation
Elgg is a registered trademark of Thematic Networks.
Cover image by RaĆ¼l Utrera is used under Creative Commons license.
Icons by Flaticon and FontAwesome.