ORA-19881: Corrupted space bitmap for datafile string, block string
Cause: When reading a space bitmap block to use Unused Block Optimization for the backup of the datafile, the space bitmap block had corrupted information.
Action: The corrupt space bitmap must be fixed before Unused Block Optimization can be used for this file. Note that you should fix the corruption in any case because it will affect the availability of the data in the file, beyond just taking backups.