Windows – Task Scheduler returns Directory Name Invalid for valid path

I'm assuming that Z: is a network drive.

Scheduled jobs are (usually) run under a System account. Network drives aren't usually mapped for System. Even if you change it to run under your credentials (or any specific user credential), you just can't count on network shares being accessible. You'll need to either:

1. Run the second script using a UNC path (and ensure that the system account has access to the UNC path)
2. Map the network drive early on in your script (in PowerShell 3 you could use New-PSDrive, but), the simplest way is to run the NET USE Z: \\SERVER\SHARE command. Of course, you might need to add user credentials: password /USER:domain\user and you could also try adding /SAVECRED /PERSISTENT:YES and running that command once and hoping it sticks (but I'd still test-path and remap it if that's an option).
3. Safest option: make a copy of the file locally.

Have you already tried navigating to C:\Windows\System32\Tasks to see if it appears there? If it isn't visible, try performing a search within that folder hierarchy. Just to double-check, is it possible that the task name you chose matches a default/built-in Windows task that already exists?

Also, please ensure that you proceed with caution. Anything you do within that folder hierarchy will have a tangible impact upon your Windows operating system.

Any task within that folder hierarchy should appear in your Task Scheduler Library. If it doesn't and you find your problematic task in C:\Windows\System32\Tasks, you should be able to delete it from that folder and recreate it accordingly.