The error is caused when App Portal attempts to load an invalid Admin security mapping or Catalog Security mapping stored in the App Portal database. This can happen when an Active Directory user or group that is currently mapped in the database is no longer valid in Active Directory.

To workaround the issue, make a backup copy of the database and run the following SQL queries against the App Portal database to remove all the current security mappings. You can then repopulate the security mappings with valid Active Directory entries.

If the error occurs under the Admin -> Site Management -> Admin Security tab, run the following SQL query:
delete from WD_NodeSecurity

If the error occurs under the Admin -> Site Management -> Catalog Security tab, run the following SQL query:
delete from WD_CatalogSecurity

After the manual database update, do an IISRESET on the App Portal Server.
Log in to the App Portal site and repopulate the security mappings with valid Active Directory groups/users.