what is required in "db.mysqldump" and "db.mysql" fileds?
The path to invoke the mysqldump or mysql programs (or just the executables if they're on the Mango user's $Path). The error,
backupMysqlWithOutDatabase:442) - java.io.IOException: Cannot run program "mysqldump"
Indicates mysqldump is not on the path.
what's the problem and how to solve it?
That issue appears to have been solved in version 3.1.3 of the excel reports module. You will probably needs to work around it by manually deleting from the excelReports table and deleting from Mango/web/modules/excelReports/web/report-data/ as appropriate. It may only be one report with a null filename, you could perhaps investigate the excelReports SQL table to see.