Please see https://github.com/FreeRDP/FreeRDP-Sharp/issues/1. If you pull the package from NuGet and use it in a project it will work fine, but compiling the solution (to e.g. produce your own package) fails. I've had similar issues with C# projects before but never got anywhere.
The line the debugger references contains a name in the DllImport statement that makes it look like they were building from Linux. The name is "libfreerdp." There is a FreeRDP NuGet package that just has the DLLs in it - using that, the names are "freerdp" and "freerdp-server." Unfortunately changing the name leads to missing symbol errors as the bindings try to call a FreeRDP winsock setup function.
Does anyone used C# enough to be able to help?
My main interest in this C# binding is that FreeRDP is barely documented itself. Ultimately I may have to write my own wrappers for it.