Error 18029 in EngageOne Vault

Vault 5.3, 5.4, 5.5, 6.0, 6.1, 7.0, 7.1, 7.2, 7.3, 7.4
The error will often be seen in when trying to run the .UNINDEX command on a file:

6:49:49 start unindexing document file [docdata\20041129-somedocument.drd]
         0    10   20   30   40   50   60   70   80   90   100
         |    |    |    |    |    |    |    |    |    |    |
         16:49:51 ERROR 18029: key [09730742V_20041129-somedocument-17-39-45_] could not be deleted in index [index\All\NIF.dri]
16:49:51 ERROR 18029: key [09730742V_20041129-somedocument-17-39-45_] could not be deleted in index [index\xxxxx\NIF.dri]
16:49:53 ERROR 18029: key [28381882C_20041129-somedocument-17-39-45_] could not be deleted in index [index\aaaaa\NIF.dri]
16:49:53 ERROR 18029: key [28381882C_20041129-somedocument-17-39-45_] could not be deleted in index [index\bbbbb\NIF.dri]
this means the unindex process tried to delete the key but did not find it. 
 
There are a number of common causes: 
- The original load was interrupted and documents past a certain point in the file weren't added resulting in these errors on unindex 
- The whole file has either not been indexed or was previously unindexed and all documents generate this error
 
To remove these files from the Vault, it may be better to use the .REMOVE command instead of .UNINDEX, because .REMOVE will move the files to the "removed" directory on the EngageOne Vault server as well as unindexing them, leaving less confusion about the state of the files.
UPDATED:  July 16, 2019