I will start by saying that the lack of a backup failure notification in this case is a bug - I've created the appropriate cases in our system to get that fixed, my apologies.
Beyond that, I think the issue here is caused by the Site Manager system being focused on local backup rather than backup of external resources - the Agent on your computer is running as the computer's SYSTEM account and therefore has no access to a mapped drive or share credentials for any other user.
The Agent attempts to validate the paths it's been given in the File and Folder backup definition (M:\ or \\SecurendoNAS\Media) and neither of them work as the share mapping or network credentials have been established for another user (Whichever one is used for interactive logon) and not for SYSTEM. When the Agent detects that there are no accessible source paths for the File and Folder backups, it exits early with an error. The notification system is not picking this up correctly and reporting the error details forward - this will be fixed in the next release.
In this case, there are a few options for making the backup work, though they are all a little 'hacky':
1. Enable anonymous read only access to the share for backup purposes - obviously this has some security issues
2. Ensure that the SYSTEM account has access to the share - on a domain, the system account (DOMAIN\COMPUTERNAME$) is a member of the Domain Users group, so if your NAS is domain integrated, you could allow access from this account/group
3. Map the share in the SYSTEM account - if you run a command line as the system account using something like SysInternals psexec, you can map the share to a drive letter for the system account using "net use" with the /PERSISTENT flag. You can also use the Windows task scheduler to achieve this - https://superuser.com/questions/753400/map-network-drive-for-the-system-user-in-windows