"ORA-01017: invalid username/password" while connecting to an instance remotely as sysdba

I'm trying to connect to a remote instance as sysdba - however, I encounter the error: "ORA-01017: invalid username/password; logon denied".

$ sqlplus "sys/xxxx@:1521/sid1 as sysdba"

SQL*Plus: Release 11.2.0.4.0 Production on Tue Sep 29 04:07:34 2015

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

ERROR:
ORA-01017: invalid username/password; logon denied

When I try to connect to another instance on the same server, I can get through:

$ sqlplus "sys/xxxx@:1521/sid2 as sysdba"

SQL*Plus: Release 11.2.0.4.0 Production on Tue Sep 29 04:07:23 2015

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>

Can someone point me to what I may have missed here?

Seems simply a password issue. Different instances may have different passwords, the SYS password is not necessarily the same for all the instances on the same server. Make sure remote_login_passwordfile is configured properly in the instances, and the password file exists for instance SID1 on the server in ORACLE_HOME/dbs/orapwSID1 (Linux-Unix) or ORACLE_HOME/database/pwdSID1.ora (Windows).

If you do not know the correct SYS password, you can recreate the password file, for example:

orapwd file=$ORACLE_HOME/dbs/orapwsid1 password=xxxx force=y

StackExchange Database Administrators Q#116445, answer score: 7