SSRS / Reporting Services - Custom Authentication - Cannot add / edit data sources

I have a reporting services server which uses custom authentication. I use this tutorial to setup my custom authentication.

https://github.com/microsoft/Reporting-Services/tree/master/CustomSecuritySample

I think the custom authentication works fine since I can register and log the user and my ASP.NET apps can access the reports with reportviewer, at least for now using an empty report as test.

My problem is, when I logged in as administrator, I cannot add a new data source, when I click New Data Source, it redirected to the page to created the data source, but a message pops up,

"An error has occurred. Something went wrong. Please try again later"

and I can't click anything on the page. When I check the log on the server, I got this,

rshost!rshost!1cb0!12/21/2020-12:52:51:: e ERROR: ReadCallback(): failed to read in read callback.
rshost!rshost!1cb0!12/21/2020-12:52:51:: e ERROR: Failed with win32 error 0x03E3, pipeline=0x0000022FBF3C2040.
rshost!rshost!2304!12/21/2020-12:52:51:: e ERROR: HttpPipelineCallback::ReadEntityBody(): failed reading data.
rshost!rshost!2304!12/21/2020-12:52:51:: e ERROR: Failed with win32 error 0x03E3, pipeline=0x0000022FBF3C2040.
library!ReportServer_0-1!2304!12/21/2020-12:52:51:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: RsWorkerRequest::ReadEntityBody., Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: An internal or system error occurred in the HTTP Runtime object for application domain ReportServer_SSRS_0-1-132529999602989514. ---> System.Runtime.InteropServices.COMException: The I/O operation has been aborted because of either a thread exit or an application request. (Exception from HRESULT: 0x800703E3)
   at Microsoft.ReportingServices.HostingInterfaces.IRsHttpPipeline.ReadEntityBody(Byte[] buffer, Int32 offset, Int32 size)
   at ReportingServicesHttpRuntime.RsWorkerRequest.ReadEntityBody(Byte[] buffer, Int32 offset, Int32 size)
   --- End of inner exception stack trace ---;

Am I doing something wrong or have I missed a step? I have checked the documentation over and over again and can't find anything that I could have missed. Thanks in advance.