For those who ran into the same issue:
I got it working by deleting the package and then re-adding it. This then saved the file without the +build.0 now everything is working.
(Note: you might want to go into the physical HD location and manually delete the old file since it isn't automatically deleted for you.)

I think the problem is that the package is being saved as: D:\...\ProxyKit.2.3.3+build.0.nupkg but the server is trying to access: d:\...\ProxyKit.2.3.3.nupkg which doesn't exist. I believe this because when I click on "Download Package > with source" I get the following response:
Could not find file 'D:...\ProxyKit.2.3.3.0.nupkg'.

Okay, we upgraded to version 5.3.11 (Build 37) and we are still getting the a 500 (Internal Server Error).

Please add ProxyKit.2.3.3+build.0 (found here) to your environment and test to see if you can duplicate our issue.

Thank you.

Okay,
Will do so. Thanks.

ProGet version: 5.0.10
Nuget version: 4.6.2.5055

We can download manually (from proget) but not through nuget. Here's the output:

C:>nuget install ProxyKit -Version 2.3.3 -Source http://xxxx.xxxx.xxx/nuget/Empower8/
Feeds used:
C:\Users\xxxx.nuget\packages
http://xxxx.xxxx.xxx/nuget/Empower8/

Attempting to gather dependency information for package 'ProxyKit.2.3.3' with respect to project 'C:\Users\xxxx', targeting 'Any,Version=v0.0'
Gathering dependency information took 193.46 ms
Attempting to resolve dependencies for package 'ProxyKit.2.3.3' with DependencyBehavior 'Lowest'
Resolving dependency information took 0 ms
Resolving actions to install package 'ProxyKit.2.3.3'
Resolved actions to install package 'ProxyKit.2.3.3'
Retrieving package 'ProxyKit 2.3.3' from 'http://xxxx.xxxx.xxx/nuget/Empower8/'.
GET http://xxxx.xxxx.xxx/nuget/Empower8/package/ProxyKit/2.3.3+build.0
NotFound http://xxxx.xxxx.xxx/nuget/Empower8/package/ProxyKit/2.3.3+build.0 504ms
WARNING: Install failed. Rolling back...
Executing nuget actions took 567.51 ms
Package 'ProxyKit.2.3.3' is not found on source 'http://xxxx.xxxx.xxx/nuget/Empower8/'.

How do we fix this issue?
Thank you.