Hello,
In order to backup Open files during a backup, you need to install the Advanced open file option. However in case you do not have the option and do not want to backup open files, you can peform the following steps:
1. From the backup job properties pane,under parameters, click on advanced options
2. There you shall find the option 'backup open files when Advanced open file option is used'
3. Choose the tab 'never' and verify the results.
Regards.