2370792 - Background job RDDIMPDP could not be started or terminated abnormally during Upgrade

Upgrade is hanging because job RDDIMPDP cannot start.

You can find the following error recorded in <DIR_PUT>/ABAP/log/SLOG:

WARNING:       Background job RDDIMPDP could not be started or terminated abnormally.
      Please check that the R/3 system is running.
      Please check the system. Use transactions SM21, SM37, SM50.
WARNING:       (This warning is harmless if no further warnings follow.)
WARNING: System PBW. Warning.        20160925020304 :
WARNING:       Background job RDDIMPDP could not be started or terminated abnormally.
      Please check that the R/3 system is running.
      Please check the system. Use transactions SM21, SM37, SM50.
WARNING:       (This warning is harmless if no further warnings follow.)

.....

In SM37, job RDDIMPDP is with status "released", but not started.

In SM21 lots of error messages can be found as below:

<Date> <Time>  <Instance Name> DIA <Process No.> <Client NO.> SAPSYS  EKL   Bckgrnd def.srvr grp SAP_DEFAULT_BTC does not contain any active bckgrnd srvrs

ABAP NetWeaver based systems

"Background job RDDIMPDP could not be started", server group, default server group, SM61, SAP_DEFAULT_BTC, upgrade, downtime phase , KBA , BC-UPG-TLS-TLA , Upgrade tools for ABAP , BC-CCM-BTC , Background Processing , Problem