It's helpful if you quote the version number also (x.x.x.x - see title bar or taskbar button). But anyway...
Try it with a UNC path instead of the mapped drive - as per paragraphs 2-onward of this article, drive mappings do not generally work when run on a Schedule (they may work if the user who mapped the drive(s) is also logged on via the console at the time, but definitely won't if he is not)
Plus, your error message suggests you have credential issues, which are sometimes related to the security token Windows issues to a Windows Scheduled Task - try adding the credentials to the Network settings page of the actual profile (you will definitely need to use a UNC path as Source to try this, or the relevant section will not be enabled).
You should also check that you do not have the option 'do not store password' checked in the Scheduled Task, as that will kill network access for sure (by issuing a lower-grade security token to the Task)
Note that Windows does not permit simultaneous access to the same \\SERVER using two or more sets of different credentials at once from the same client PC (whether same shares or different). If your Z drive is mapped with the credentials of user-X, Windows will either block an attempt to connect to the server with the credentials of user-Y, or will silently re-use the X credentials/connection, which may not have the correct privileges for that share (etc)
Try it with a UNC path instead of the mapped drive - as per paragraphs 2-onward of this article, drive mappings do not generally work when run on a Schedule (they may work if the user who mapped the drive(s) is also logged on via the console at the time, but definitely won't if he is not)
Plus, your error message suggests you have credential issues, which are sometimes related to the security token Windows issues to a Windows Scheduled Task - try adding the credentials to the Network settings page of the actual profile (you will definitely need to use a UNC path as Source to try this, or the relevant section will not be enabled).
You should also check that you do not have the option 'do not store password' checked in the Scheduled Task, as that will kill network access for sure (by issuing a lower-grade security token to the Task)
Note that Windows does not permit simultaneous access to the same \\SERVER using two or more sets of different credentials at once from the same client PC (whether same shares or different). If your Z drive is mapped with the credentials of user-X, Windows will either block an attempt to connect to the server with the credentials of user-Y, or will silently re-use the X credentials/connection, which may not have the correct privileges for that share (etc)