For some reason I couldn’t delete credentials from Veeam due to them being used on a Shared Folder that did not existing in a backup repository or Tape to Folder. In the end I had to use SQL remove this.

1. Verify the record to be deleted by running the following query against the configuration database (VeeamBackup by default);
   
   SELECT * FROM [backup.model.mrulist]
2. Run the following query, changing <share path> to match the record to be deleted.
   
   DELETE FROM [backup.model.mrulist] WHERE url = ‘<share path>’