All of the log files created by Exchange since the last full backup need to be
present for an incremental backup to succeed.

Try a full backup and then the incremental again

Geoff Andrews wrote:

> Could somebody please help me, When I try to backup my "Exchange MDB Database",
> I get an "event id 57920" error with the message "Error message: Unable to
> perform an incremental backup because a required Microsoft Exchange database
> log file could not be found.". Is there something i can do to cure this??
>
> Thanks in advance