Upload fails, wants "target"

[Guest]

I have both v4.8.9 and v5.0.8 installed on 2 VMs.
I have a small .upack file and a upack.json file on a third VM

when I issue (to the v4.8.9 server)
C:\Users\build-auto\Desktop>upack.exe push c:\Users\build-auto\Desktop\MYPKG-1.2.3.upack http://<489PGserver>:81/upack/myFeed/upload --user=usr:pw
Package: installer/MYPKG
Version: 1.2.3
installer/MYPKG 1.2.3 published!

BUT when I issue same to v5.0.8 server:
C:\Users\build-auto\Desktop>upack.exe push c:\Users\build-auto\Desktop\MYPKG-1.2.3.upack http://<508PGserver>:81/upack/myFeed/upload --user=usr:pw
Package: installer/MYPKG
Version: 1.2.3
Unhandled Exception: System.Net.Http.HttpRequestException: Response status code does not indicate success: 500 (Internal Server Error).

When I look in the server error logs, I see:
Value cannot be null. Parameter name: target (plus lots of details)

What is "target"??? I do not see anything in the Native API, nor the upack docs. I've tried searhing the forums, but so far no luck.

Product: ProGet
Version: 5.0.8

[ben]

Hello Bruce,

It looks like this can happen when the package store fails to open the file for writing.

If you don't have a custom package store set for the UPack feed, make sure Storage.ProGetPackagesLibrary is set to a full path in /administration/advanced-settings and that the ProGet web server process can modify files in that path.