ORA-01126: database must be mounted in this instance and not open in any instance.

Ran into this one yesterday and wanted to share the solution since searches for ORA-01126 on MOS and Google did not reveal this particular solution.

We received this error when we were trying to take a database (not a production database!) out of archivelog mode for a large data load.  This is a single instance database...no RAC involved.  We set the ORACLE_SID and logged in "sys / as sysdba" on the database server.  After several "shutdown immediate, startup mount, alter database noarchivelog" attempts, we tried starting up in restricted mode which made no difference...the same error occurred.

We have standby databases in our environment but this was not one of them, or so we thought.  Using dgmgrl, we logged in to the database and issued "show configuration".  Sure enough, there was a configuration there with the database in question listed as the primary with no standbys listed.  Apparently we had fooled around with this database coupled with a standby in the past...several years ago...but the configuration remained.  We issued "remove configuration" and logged off dgmgrl after which we successfully put the database in noarchivelog mode.