ORA-12528: TNS:listener: all appropriate instances are blocking new connections

LSNRCTL>lsnrctl status

Service "orcl" has 1 instance(s).
Instance "orcl", status BLOCKED, has 1 handler(s) for this service...

SQL> conn scott/tiger@orcl

ERROR:
ORA-12528: TNS:listener: all appropriate instances are blocking new connections

$rman target sys/password@PRODDB auxiliary sys/password@TESTDB

RMAN-04006: error from auxiliary database:
ORA-12528: TNS:listener: all appropriate instances are blocking new connections

SOLUTIONS

The best place to start with is to check the tnsnames.ora and listener.ora files and make sure that all is well.

1. By passing oracle Net authentication  using operating system authentication for auxiliary database.

$export ORACLE_SID=TESTDB
rman target sys/password@PRODDB auxiliary /
instead of 
rman target sys/password@PRODDB auxiliary sys/password@TESTDB

2. But if still Oracle Net authentication is required put UR=A entry in the tnsnames.ora file to take advantage of using restricted session privilege across the network.

3.  To statically configure service information for the listener put the following entry in the listener.ora file  
     (SID_DESC=
        (GLOBAL_DBNAME=TESTDB)
        (SID_NAME=TESTDB)
        (ORACLE_HOME=/oracle/base/dbhome01/)
      )