I have created a symbolssource that is being used for a while already.
If I try to push packages generated in the old VS, it pushes the packages correctly, if I try to push new packages I got this error:
nuget push *.symbols.nupkg 123 -Source http://localhost:3182/NuGet Empurrando MYPACKAGE para 'http://localhost:3182/NuGet' ... Failed to process request. 'Package submission failed: Unknown custom metadata item kind: 6. See http://www.symbolsource.org/Public/Home/Help for possible reasons. Fiddler may help diagnosing this error if your client discards attached detailed information.'.
I tried to update the symbolssource with the latest nuget packages, but still I have the same issue.
The problem here was the MSBuild 14 with the pdb metadata. There is a fix for it in this link.