UWP build fails: System.Xml cannot be found

Hello,

I am trying to build my project for UWP, HoloLens to be specific. I am getting XML data from the web and parse that using NewtonSoft.JsonConvert.SerializeXmlNode (XmlNode node) (returns string). Here's a list of stuff you need to know and stuff that I tried already:

• My Unity version is 2018.1.3f1, Scripting Runtime Version is .NET 4,x Equivalent, Scripting Backend is Mono, API Comp. Lvl is .NET 4.x as well.

• The exact error message: Assets\Scripts\RestManager.cs(62,23): error CS7069: Reference to type 'XmlNode' claims it is defined in 'System.Xml', but it could not be found

• I double checked the necessary includes and also checked for a similar error described here: https://github.com/OmniSharp/omnisharp-vscode/issues/1986

What I want to do works both in a standalone build and the editor.

Any help is much appreciated!