There are two possibilities for this issue:
1) the Developer Portal had been localized at some point in time. Recently the localization has been removed.
This is a defect in the Portal software that will be fixed in Portal 3.5 CR10.
To correct this problem please use the attached file and the following steps:
Go to http://site/admin as an administrator
Select Content Items from top left menu
Navigate to /sitebuilder/content/groups/Metadata
Select Advanced -> Edit Text Translation from the ApplicationPlatforms.xml content item menu
Copy the XML text into the clipboard and save on desktop
Select Advanced -> Delete from the ApplicationPlatforms.xml content item menu
Apply the patch attached to this article
Return to /sitebuilder/content/groups/Metadata
Select Advanced -> Edit Text
Paste the content that was saved earlier
Select Publish from the ApplicationPlatforms.xml content item menu
2) The error is indicating that there is an issue with the Portal's search index. We recommend the following:
sudo service apiportal stop
sudo mv /opt/Deployments/lrs/repository/INDEX /opt/Deployments/lrs/repository/INDEX.abcde
sudo service apiportal start
This will rebuild the Portal's search index.