Have you any 3rd party installed that is not fully compatible with Elgg 1.8 - test by disabling all 3rd party plugins to see if the issue remains.

Have you followed the additional upgrade instuctions in UPGRADE.txt for upgrading from Elgg 1.7.X to Elgg 1.8.X, i.e. deleting the listed folders and plugins with obsolete files / views. If not, a deprecated view could interfere and result in such an issue you described. If you missed to remove the folders I would suggest to clean up your Elgg install directory by removing ALL files and subfolders with the following exceptions: any 3rd party plugin folders in mod, the .htaccess file and the settings file in enginge/settings.php. Everything else can go (assuming you haven't modified any core files). After deleting the files install the latest Elgg version on your server again and run upgrade.php. Then you have no deprecated files remaining.

Also, you need to replace the content of your "Elgg 1.7" .htaccess with the content of htaccess_dist that came with the Elgg 1.8 version. If you made any changes in .htaccess on your own, you need to re-add them to the "new" .htaccess file. This upgrade of the .htaccess file might also help with your problem if the other two possibilities did not work.