Unable to push packages to universal feed

I'm running a free version of ProGet Version 5.1.19 (Build 7) on a Windows Server 2016 Standard machine. The ProGet instance has been installed in IIS and uses a local SQL Express database.

Most everything seems to function correctly, except I am unable to push packages to a universal feed. When I run upack.exe push, the package name and version are printed to stdout and then the command just blocks there seemingly doing nothing. There are no error or warning messages to be found in the proget web interface nor in the iis logs.

For example:

upack.exe --version
upack 2.2.3.9
Usage: upack «command»
<snip>

upack.exe push "C:/Path/To/TestApplication-0.1.0.upack" "http://proget.mycompany.local:8000/upack/Test-Packages" --user=api:[RedactedMySuperSecretApiKey]
Package: MyCompany/Sandbox/TestApplication
Version: 0.1.0-ci.33
^C   # Blocks here so I eventually cancel the command...

Any idea what might be wrong?

Product: ProGet
Version: 5.1.19

Further investigation shows this to be a regression in upack 2.2.3.9.
When using upack 2.2.2.2 publishing functions correctly:

upack.exe" --version
upack 2.2.2.2
Usage: upack «command»
<snip>

upack.exe push "C:/Path/To/TestApplication-0.1.0.upack" "http://proget.mycompany.local:8000/upack/Test-Packages" --user=api:[RedactedMySuperSecretApiKey]
Package: MyCompany/Sandbox/TestApplication
Version: 0.1.0-ci.33
MyCompany/Sandbox:TestApplication 0.1.0-ci.33 published!

Reported as https://github.com/Inedo/upack/issues/49

This should be fixed in v2.2.4, but the issue did not get added to GitHub - I'll correct that.