When we tried to connect to the database, it failed with ORA-12526 like the following.
SQL> conn hr/hr@orcl
ORA-12526: TNS:listener: all appropriate instances are in restricted mode
ORA-12526 means that the database is in a very special mode which restrict all external connections except SYS from accessing the database. It's called restricted mode.
For users privileged with RESTRICTED SESSION like SYS or SYSTEM who want to remotely connect to the database in restricted mode, you should configure a static service in listener by adding a SID_LIST_LISTENER entry for them.
The way we solve ORA-12526 is just like we did in how to connect a restricted database.