Message Oracle ORA-38740 - Usable blocks value string is not valid.

ERRORUsable blocks value string is not valid.
CAUSEA flashback database log file header contained a usable blocks value greater than the file size. The flashback database log file file is corrupt.
ACTIONIf you are doing a FLASHBACK DATABASE, then the flashback cannot be performed because of the corrupted log. A restore and an incomplete recovery must be performed instead.