after we install VS2015 in our server our symbol packages are getting this error when we try to push then to the symbols server: (506) Package submission failed: Unknown custom metadata item kind
I didn't find a reason for it, then I create another csproj from scratch, then it worked partialy.
When the PDB is added to the package it fails when I try to push the package to the symbol source.
The problem here was the MSBuild 14 with the pdb metadata. There is a fix for it in this link.