API v2 .NET, problem with upload session

Dropbox.Api.BadInputException: Error in call to API function "files/upload_session/finish": HTTP header "Dropbox-API-Arg": could not decode input as JSON
at Microsoft.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at Microsoft.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at Microsoft.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at Dropbox.Api.DropboxRequestHandler.<Dropbox.Api.Babel.ITransport.SendUploadRequestAsync>d__5`3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Archivist.UploadForm.<ChunkUpload>d__9.MoveNext() in c:\Dropbox\Archivist\Archivist\UploadForm.cs:line 205
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Archivist.UploadForm.<Upload>d__5.MoveNext() in c:\Dropbox\Archivist\Archivist\UploadForm.cs:line 152
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Archivist.UploadForm.<UploadForm_Shown>d__0.MoveNext() in c:\Dropbox\Archivist\Archivist\UploadForm.cs:line 72
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__4(Object state)

************** Loaded Assemblies **************
mscorlib
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/mscorlib.dll
----------------------------------------
Archiviste
Assembly Version: 1.0.0.0
Win32 Version: 1.0.0.0
CodeBase: file:///C:/Dropbox/Archivist/Archivist/bin/Release/Archiviste.exe
----------------------------------------
System.Windows.Forms
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Windows.Forms/v4.0_4.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Drawing/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
System
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34238 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System/v4.0_4.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Configuration
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Configuration/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Configuration.dll
----------------------------------------
System.Xml
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34234 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Xml/v4.0_4.0.0.0__b77a5c561934e089/System.Xml.dll
----------------------------------------
System.Net.Http.WebRequest
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Net.Http.WebRequest/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Net.Http.WebRequest.dll
----------------------------------------
System.Net.Http
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Net.Http/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Net.Http.dll
----------------------------------------
System.Core
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209 built by: FX452RTMGDR
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Core/v4.0_4.0.0.0__b77a5c561934e089/System.Core.dll
----------------------------------------
Dropbox.Api
Assembly Version: 0.1.5805.22713
Win32 Version: 0.1.0.0
CodeBase: file:///C:/Dropbox/Archivist/Archivist/bin/Release/Dropbox.Api.DLL
----------------------------------------
System.Threading.Tasks
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Threading.Tasks/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Threading.Tasks.dll
----------------------------------------
Microsoft.Threading.Tasks
Assembly Version: 1.0.12.0
Win32 Version: 1.0.168.0
CodeBase: file:///C:/Dropbox/Archivist/Archivist/bin/Release/Microsoft.Threading.Tasks.DLL
----------------------------------------
Newtonsoft.Json
Assembly Version: 7.0.0.0
Win32 Version: 7.0.1.18622
CodeBase: file:///C:/Dropbox/Archivist/Archivist/bin/Release/Newtonsoft.Json.DLL
----------------------------------------
Microsoft.Threading.Tasks.Extensions
Assembly Version: 1.0.12.0
Win32 Version: 1.0.168.0
CodeBase: file:///C:/Dropbox/Archivist/Archivist/bin/Release/Microsoft.Threading.Tasks.Extensions.DLL
----------------------------------------
System.Net
Assembly Version: 4.0.0.0
Win32 Version: 4.0.30319.34209
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Net/v4.0_4.0.0.0__b03f5f7f11d50a3a/System.Net.dll
----------------------------------------