Well, it so happens that I started a recently upgraded 126.96.36.199 DB and got this error. It also meant that the DB wouldn’t mount.
It turns out that the default ORA_NLS10 variable to set to a wrong value (I am suspecting EBS autoconfig, but this research is still pending). Setting it to the correct value fixed the issue for me. I also updated the EBS environment setting script with the correct value.
ORA_NLS10=$ORACLE_HOME/nls/data export ORA_NLS10