Do you have the file nav.php in mod/members/views/default/members/.
If you do, you must delete it.
I had the same problem as Scotty57 after upgrading to 1.9. As per the advice of Per Jensen, I removed nav.php in mod/members/views/default/members/ and the members plugin now functions correctly. Any insight into why this fixes the issue?
Edit
It appears the issue with having nav.php might be found in mod/members/pages/members/index.php at line 8
This file was part of Elgg 1.8 but is no longer included in Elgg 1.9. Basically, any core file that is no longer part of the Elgg version a site gets upgraded to but remains on the server because you only copy the new version over the old version could result in problems. Therefore, I would always remove the files of the old version completely before installing the new version. As Juho suggested, you only need to keep .htaccess and settings.php from the old Elgg version and of course you can keep your 3rd party plugins in mods.
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