Which Elgg version?
Have you tested with 3rd party plugins disabled to see if any of them might cause the problem?
Or maybe something is broken in the files of the externalpages plugin on your installation. Maybe it helps to copy this folder again to the server (best would be to delete the old plugin folder to make sure that no outdated files remain if you had ever used an older version of Elgg before).
info@elgg.org
Security issues should be reported to security@elgg.org!
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