Yesterday I found a way to get the backups running again. I made a downgrade of client and NMDA and this solved the issue. It seems to be a bug in this version. Unfortunately also Dell support couldn't deliver a solution as they did not know this problem.  Error "The fetchss_range() function did not return a valid  list" is definitely caused by NetWorker.  
Hello all, we upgraded NW client and NMDA package from 8.5 to 19.6.1.2. Oracle server with an Impax system is running in a 2 node cluster. During upgrade to the newer NetWorker version the nmda_oracle.cfg was overwritten and something more happened, but I don’t know what. After upgrade no backup was possible anymore. Neither database backup nor redo logs. I found out that I need to copy the ibnsrora.so into the libobk.so file. Linking libnsrora.so into libobk.so didn’t help. Then the redo log backup ran successfully again. But the database backup still fails with this error. I searched with Google and also on Dells support pages, but without success. Also adding *@* in remote access didn’t help as this was missing. Please can someone help to find a solution? Thanks a lot. Starting backup at 27-SEP-22 channel t1: starting full datafile backup set channel t1: specifying datafile(s) in backup set including current control file in backup set channel t1: starting piece 1 at 27-SEP-22 released channel: t1 RMAN-00571: =========================================================== RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS =============== RMAN-00571: =========================================================== RMAN-03009: failure of backup command on t1 channel at 09/27/2022 06:47:41 ORA-27191: sbtinfo2 returned error Additional information: 3282    The fetchss_range() function did not return a valid list.  (100:13:11)   Recovery Manager complete. The backup process failed: RMAN exited with return code '1'. Exiting with error: RMAN exited with return code '1'. impaxdb:     RMAN exited with return code '1'.
sbtinfo2 The fetchss_range() function did not return a valid list