GenerateTokenInGroupAsync returns BadRequest if used in GenerateTokenRequest with customData

I have the challenge, want to host a PowerBI report embedded where the Row filter is to be implemented via the CustomData feature.

When I use the CustomData property, I have the problem that when calling the GenerateTokenInGroupAsync function, it always throws a BadRequest error message.

When I call the GenerateTokenInGroupAsync function without a CustomData property I get back a valid result, only the report is not filtered.

Code

GenerateTokenRequest generateTokenRequestParameters = new GenerateTokenRequest(
                        accessLevel: "View",
                        identity: new EffectiveIdentity(
                            username: PowerBiAuthConstants.Username,
                            datasets: new List<string> { reportConfig.DatasetId },
                            roles: new List<string> { "Username" },
                            customData: fgnr));


var tokenResponse = await client.Reports.GenerateTokenInGroupAsync(
                        groupId: reportConfig.GroupId,
                        reportKey: report.Id,
                        requestParameters: generateTokenRequestParameters);

Exception

Massage: Operation returned an invalid status code 'BadRequest'

StackTrace:   at Microsoft.PowerBI.Api.V2.Reports.<GenerateTokenInGroupWithHttpMessagesAsync>d__22.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 Microsoft.PowerBI.Api.V2.ReportsExtensions.<GenerateTokenInGroupAsync>d__55.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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at KIND.PowerBI.WebApi.DataAccess.PowerBiAdapter.<CreateEmbedConfig>d__0.MoveNext() in D:\TFS\PH1\Dev\services\KIND.PowerBI.WebApi\KIND.PowerBI.WebApi\DataAccess\PowerBiAdapter.cs:line 100