Web API 2: Random 401 unauthorized on POST

I'm currently building an Angular 2 + ASP Web API 2 application and came across an annoying authorization issue.

I can create an account and log in using the standard identity framework without any problem using the bearer token, but when I'm trying to POST data to my controller I have a 401 authorization ... which occurs only on the second call.

Here is for example the first POST which worked perfectly fine:

Accept:application/json, text/plain, */*
Accept-Encoding:gzip, deflate, br
Accept-Language:fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4
Authorization:Bearer 2x4FSZqJ6Msos870_gIl4aKjgdms1PEGNnFp2ptM6Rrgs4vtmvnMdu2nzEfBoly15CI2bQss5DVe-bkN2uSTHrMP7F6blK90DcFt095xTsWk3BJ_5RiZ-jsXOrqTZaisChIbWgGN0o-DiTEA_ojFKImgsX9yip6hloZ6GI_Cd0eg6EjX6S_PUmmyI13oiBAHKROmDvVoB4y0-DbHPnAO--x9yGxU1z_SRwFYqX8Dua7oAvpbyl2VFIqqA39DlQ0E9JPaRC0gvrBxeS-nibAaBeDUwLSAQm6HOe-vynVhQeGrBvHl7r3gicaNdaS5UZvrC43KFTe6__wR1aDaIgpMejlp-eVnjTVbcxvR16XOXrud1W-tNcoOHtoMdGKKb0IvfxK_GCety5eiiTIGWUpA26nF5cCZEIna8ZJawXRiBZVV__MEPDBlR68mJHvHVKfm5w_jupwF5_oehcKwbT_QZ92hxg4UV8uUaiisqbIe7jQ
Connection:keep-alive
Content-Length:60
Content-Type:application/json
Cookie:.AspNet.Cookies=wcZzztFwOl41pBDgXshPnYCaaQhULRqu9O6grYPDhUx4cSY8PRY1oBRQLs3gPz4ySoxQBaaehtCLDeOFzXAN7q_UMbZps82aCajwvBQewtu_SLizCRTU9UHncWy0EFnJtLAuF5u_8sKW6sNNPTHfDtmjl3UVQkkvYDBceJC5F-sISGqH-sPFwEGmoXgcKLHWfPlejxAvRCRvGbhFhrdKpk_sycoi_B0sBe9Kc8EULXlybEeUolyyrY7L5HvOUIuujLThILt6ipYEmIk8b_2x32uCq7euh5Y_RDzI009SMceOSBs6HYsqUxz6lR2F3KvlcYBQ3rDr8qALhJBnMyJsysdDIruF9dVjK7-IjdEBPXMsGCnHK5gQs_1bIflSaPBbxVPn2VzPui-WChDCdoVXRgGRRnaEtkzaTOhBZIjlLZ1DWS1MqkM-V0khFQxBDqsxll0pZTgNZuwEOLBYoWWiAK9fATXLSmtMYA2UMKoAE0M
Host:localhost:56762
Origin:http://localhost:56762
Referer:http://localhost:56762/tasks/details;id=null
User-Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

With this data: {Title: "Test", Category: "Test", Entries: []}

And now the one returning a 401 error, performed a few seconds later:

Accept:application/json, text/plain, */*
Accept-Encoding:gzip, deflate, br
Accept-Language:fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4
Authorization:Bearer 2x4FSZqJ6Msos870_gIl4aKjgdms1PEGNnFp2ptM6Rrgs4vtmvnMdu2nzEfBoly15CI2bQss5DVe-bkN2uSTHrMP7F6blK90DcFt095xTsWk3BJ_5RiZ-jsXOrqTZaisChIbWgGN0o-DiTEA_ojFKImgsX9yip6hloZ6GI_Cd0eg6EjX6S_PUmmyI13oiBAHKROmDvVoB4y0-DbHPnAO--x9yGxU1z_SRwFYqX8Dua7oAvpbyl2VFIqqA39DlQ0E9JPaRC0gvrBxeS-nibAaBeDUwLSAQm6HOe-vynVhQeGrBvHl7r3gicaNdaS5UZvrC43KFTe6__wR1aDaIgpMejlp-eVnjTVbcxvR16XOXrud1W-tNcoOHtoMdGKKb0IvfxK_GCety5eiiTIGWUpA26nF5cCZEIna8ZJawXRiBZVV__MEPDBlR68mJHvHVKfm5w_jupwF5_oehcKwbT_QZ92hxg4UV8uUaiisqbIe7jQ,Bearer 2x4FSZqJ6Msos870_gIl4aKjgdms1PEGNnFp2ptM6Rrgs4vtmvnMdu2nzEfBoly15CI2bQss5DVe-bkN2uSTHrMP7F6blK90DcFt095xTsWk3BJ_5RiZ-jsXOrqTZaisChIbWgGN0o-DiTEA_ojFKImgsX9yip6hloZ6GI_Cd0eg6EjX6S_PUmmyI13oiBAHKROmDvVoB4y0-DbHPnAO--x9yGxU1z_SRwFYqX8Dua7oAvpbyl2VFIqqA39DlQ0E9JPaRC0gvrBxeS-nibAaBeDUwLSAQm6HOe-vynVhQeGrBvHl7r3gicaNdaS5UZvrC43KFTe6__wR1aDaIgpMejlp-eVnjTVbcxvR16XOXrud1W-tNcoOHtoMdGKKb0IvfxK_GCety5eiiTIGWUpA26nF5cCZEIna8ZJawXRiBZVV__MEPDBlR68mJHvHVKfm5w_jupwF5_oehcKwbT_QZ92hxg4UV8uUaiisqbIe7jQ
Connection:keep-alive
Content-Length:62
Content-Type:application/json
Cookie:.AspNet.Cookies=wcZzztFwOl41pBDgXshPnYCaaQhULRqu9O6grYPDhUx4cSY8PRY1oBRQLs3gPz4ySoxQBaaehtCLDeOFzXAN7q_UMbZps82aCajwvBQewtu_SLizCRTU9UHncWy0EFnJtLAuF5u_8sKW6sNNPTHfDtmjl3UVQkkvYDBceJC5F-sISGqH-sPFwEGmoXgcKLHWfPlejxAvRCRvGbhFhrdKpk_sycoi_B0sBe9Kc8EULXlybEeUolyyrY7L5HvOUIuujLThILt6ipYEmIk8b_2x32uCq7euh5Y_RDzI009SMceOSBs6HYsqUxz6lR2F3KvlcYBQ3rDr8qALhJBnMyJsysdDIruF9dVjK7-IjdEBPXMsGCnHK5gQs_1bIflSaPBbxVPn2VzPui-WChDCdoVXRgGRRnaEtkzaTOhBZIjlLZ1DWS1MqkM-V0khFQxBDqsxll0pZTgNZuwEOLBYoWWiAK9fATXLSmtMYA2UMKoAE0M
Host:localhost:56762
Origin:http://localhost:56762
Referer:http://localhost:56762/tasks/details;id=null
User-Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

With this data: {Title: "Test2", Category: "Test2", Entries: []}

I see no real differences between my two posts. They basically are the same. When I refresh in the browser, re-log, and do these tests again, it behaves the exact same way: 1st request returns HTTP 200, second HTTP 401.

Here is my controller action:

[HttpPost]
[Route("Create")]
[InjectUserIdInServiceFilter]
public int CreateTask(CreateUpdateTimedTaskViewModel timedTask)
{
      var model = this.mapper.Map<CreateUpdateTimedTaskViewModel, TimedTask>(timedTask);
      return this.taskService.Create(model);
}

With my controller definition being the following:

[Authorize]
[RoutePrefix("api/TimedTask")]
public class TimedTaskController : ApiController

Please note that I use the default auto-generated code for the Identity / OAuth / Owin part; I didn't alter its behavior.

Does anybody have any clue about what is happening?

Thanks in advance

T. Thomas