DDBoost: sbtbackup: dd_rman_connect_to_backup_host failed

A common error. It happens when the datadomain host or mtree is unreachable.
For the first situation, contact the OS/Network administrator. Is can be a firewall limitation, DNS miss (if using DNS hosting) or, in some cases, networks physically unreachable.

For the second case, try to [re]send user/pass to access datadomain:

Starting backup at 24-OCT-15
using target database control file instead of recovery catalog
allocated channel: ORA_SBT_TAPE_1
channel ORA_SBT_TAPE_1: SID=191 instance=almdbdw_1 device type=SBT_TAPE
channel ORA_SBT_TAPE_1: Data Domain Boost API
allocated channel: ORA_SBT_TAPE_2
input datafile file number=00001 name=+DGMYDB/almdbdw/datafile/system.267.849463017
channel ORA_SBT_TAPE_1: starting piece 1 at 22-JUL-15
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on ORA_SBT_TAPE_1 channel at 10/24/2015 10:03:50
ORA-19506: failed to create sequential file, name="a4qcme1l_1_1", parms=""
ORA-27028: skgfqcre: sbtbackup returned error
ORA-19511: Error received from media manager layer, error text:
 sbtbackup: dd_rman_connect_to_backup_host failed
channel ORA_SBT_TAPE_1 disabled, job failed on it will be run on another channel

Sending user/password to acess data domain as follow and, after that, re-run the your action.

RUN {
ALLOCATE CHANNEL t1 TYPE SBT_TAPE PARMS 'BLKSIZE=1048576, SBT_LIBRARY=$ORACLE_HOME/lib/libddobk.so,ENV=(STORAGE_UNIT=$STORAGE_UNIT,BACKUP_HOST=$DATADOMAIN_HOST,ORACLE_HOME=$ORACLE_HOME)' FORMAT '%U-%d';
send 'set username $DDBOOST_USER password $PASSWORD servername $DATADOMAIN_HOST';
RELEASE CHANNEL t1;
}

Hugs!

Matheus.