Unable to view NuGet feed in Visual Studio when feed has connector with filter

I have a NuGet feed set up with a connector to https://www.nuget.org/api/v2/.
I can view the packages within the feed in VS but when I add a filter to the connector, for example log4net, the packages are no longer displayed.
The UI within VS just displays Retrieving information...

If I look on the server running ProGet then the following exception message is logged:

Exception message: Nullable object must have a value.

Product: ProGet
Version: 3.5.5

Can you try to restart the website, and see if it goes away?

If not, then the problem would be between the ProGet server and NuGet.org. Something like a firewall/proxy that blocks/intercepts/rewrites the response and makes it unreadable.

We cannot reproduce this ... plus this a major feature of the software, and no one else has reported it :)

I suggest you use Fiddler to find the EXACT URL that is causing the problem, then try visiting that URL in your browser. Then, try running the ProGet server interactively.

I have captured the URL:

www.nuget.org/api/v2/Search()/$count?$filter=(IsLatestVersion) and ((tolower(Id) eq 'log4net')%20or%20(tolower(Id)%20eq%20'json'))&searchTerm=''&targetFramework='net451%7Cnet451%7Cnet451'&includePrerelease=false

This works via a browser but takes ~15 seconds to return a response.

This is related to the timeout question, so I've posted the answer there -- http://inedo.com/support/questions/2915#inline-post-2936