Solved! Go to Solution.
Further testing has reveiled that trying to add an entry to my blank Excludes list throws "No selections exist for this resource" error.
There appear to be some other documented errors like this though not pertaining to the Exclude list and I don't know of those solutions are of any help.
SELECT * FROM vwScriptPropertiesBackup WHERE ScriptID='10011001-1001-1001-0101-010101010101'Shows that all my Exludes are still in the BE database but I guess there is some corruption that keeps the Backup Exec application from being able to acces correctly to manage the list. But I find the Excludes are correctly applied during backups.