NetWorker: Cannot run save: Unknown action 2 requested. Rejecting request.

After an upgrade of the client's version the backup started to fail with error: 

save: Can not run save: Unknown action 2 requested.  Rejecting request.

The NetWorker client was upgraded to a newer release than the NetWorker server.
For example, the client was upgraded to 19.11 while the NetWorker server is 19.10.x.

This same error could arise for other backups, not only file system backups.
For instance, if doing an NMC backup, where NMC is on 19.11 version, and NetWorker server on 19.10 the error would be:

savepsm: Can not run savepsm: Unknown action 2 requested.  Rejecting request.

The NetWorker clients must be less than or equal to the NetWorker server version. If the NetWorker clients are on a newer release than the NetWorker server, unexpected errors are observed during backup or restore operations of those clients.

See the NetWorker Version Interoperability Report: NetWorker_Interop_Report.pdf
Additional compatibility information is available in: E-Lab Interoperability Navigator 2.0-HOME (You must sign in with your Dell support account).

Upgrade the NetWorker server to the same version as the client (or newer).
Alternatively downgrade the client to the same version as the NetWorker server.

NetWorker software and installation documentation is available on the Dell support site: Support for NetWorker | Overview (You must sign in with your Dell support account).