Try using standard dotnet pack from AppVeyor (and not copy stuff to the side in the packages folder)

ICSharpCode.AvalonEdit/ICSharpCode.AvalonEdit.csproj

@@ -7,8 +7,7 @@
     <DefineConstants>TRACE;DOTNET4</DefineConstants>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>ICSharpCode.AvalonEdit.snk</AssemblyOriginatorKeyFile>
-    <OutputPath>..\packages\lib</OutputPath>
-    <DocumentationFile>..\packages\lib\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile>
+    <DocumentationFile>ICSharpCode.AvalonEdit.xml</DocumentationFile>
     <Company>ic#code</Company>
     <Copyright>2000-2019 AlphaSierraPapa for the SharpDevelop Team</Copyright>
     <Description>AvalonEdit is the WPF-based text editor used in SharpDevelop. There are four builds of AvalonEdit included in this package. .NET 3.5, 4.0, 4.5 and .NET Core 3.0.</Description>

appveyor.yml

@@ -7,7 +7,7 @@ dotnet_csproj:
   patch: true
   file: '**\*.csproj'
   version: '{version}'
-  package_version: '{version}'
+  package_version: '{version}-preview2'
   assembly_version: '{version}'
   file_version: '{version}'
   informational_version: '{version}'
@@ -28,9 +28,11 @@ build:
   project: ICSharpCode.AvalonEdit.sln
   parallel: true
   verbosity: minimal
-
+  publish_nuget_symbols: true 
+  use_snupkg_format: true
+  
 after_build:
-  - ps: .\packages\create_nuget.ps1
+  #- ps: .\packages\create_nuget.ps1
   #- ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 
 after_test:
@@ -39,3 +41,4 @@ after_test:
 artifacts:
   - path: '**\*.nupkg'
     name: NuGet
+  - path: '**\*.snupkg'