ORA-19821: an intentionally corrupt log file was found

 If this error appears while opening your database, it means that an instance crash happened before the database start and instance recovery needs redo data to be applied to the database, but redo data is not available in the redo log file. The reason of missing data is that redo data was not written to the log file when database instance crashed. The data flushing to the redo logs was probably stopped because of setting parameter “_disable_logging” to TRUE to speed up the data loading. Alert log file would show entries similar to the following.