Hi,
To be honest I don't really know, but googling for the error I found this:
I was receiving a 406 error with only a specific package.
It turns out the package was larger than the 30MB asp.net limit set in web.config of the nuget server. Increasing the value of maxRequestLength in the httpRuntime element fixed the issue for me.
From NuGet.Server - 406 (Not Acceptable) Error · Issue #5084 · NuGet/NuGetGallery · GitHub
Could this be the case? FlexCel nupackage is 31 mb, so maybe it is the reason it is failing and the others don't?