I was having the same error:
Backup- \\servername\Microsoft Information Store\Second Storage Group V-79-57344-759 - Unable to complete the operation for the selected resource using the specified options. The following error was returned when opening the Exchange Database file: '-528 The current log file is missing. '
At first I has a random success rate of +/- 50%, and then it started to get worse, until at some point the backups always failed with the above error.
After restarting the Backup Exec Remote Agent on the Exchange server, the error went away.
I've now created a scheduled task to restart the Backup Exec Remote Agent every day.
My jobs are running successful, for a few days now.