Sharepoint hosted app, Workflow not starting "ACS50027: JWT token is invalid."

Hi all, 

In my sharepoint hosted app Worflow I get an ACS JWT Token error, saying that the token service returned and error of type 'invalid_client'. The exact same app did work one week ago and I didn't change anything since then. The workflow is triggered when I add a doc to a doc library. I can see in the Workflow overview that the Workflow is started, but nothing happens due to this error. Full error message:

Activiteit in uitvoering

Laatste verzoek wordt opnieuw geprobeerd. Nieuwe poging gepland na 11/14/2014 9:42. Details van laatste verzoek: HTTP  aan https://xyzar-4f2186066bfd1e.sharepoint.com/sites/developer/SmEQO-SharepointApp/_api/sp.utilities.utility.ResolvePrincipalInCurrentcontext?input='i%3A0%23.f%7Cmembership%7Ctomvanderhoek%40xyzar.onmicrosoft.com'&scopes='15'&sources='15'&inputIsEmailOnly='false'&addToUserInfoList='False', correlatie-id: , instantie-id: d072966d-416c-4de7-b1a8-bce7dfcfea90

System.Net.WebException: The request was aborted: The request was canceled. ---> System.InvalidOperationException: Failed to fetch an access token from the token service. The token service returned an error type of 'invalid_client' with the following description: ACS50027: JWT token is invalid.
Trace ID: a4987ebb-bbf0-4fca-9748-4f59a729b36c
Correlation ID: c30f2f03-ec95-3378-bb29-45eff2d12798
Timestamp: 2014-11-14 16:34:56Z ---> System.Net.WebException: The remote server returned an error: (400) Bad Request.
   at System.Net.HttpWebRequest.GetResponse()
   at Microsoft.Activities.Hosting.Security.OAuthS2SSecurityTokenServiceCredential.FetchAccessToken(Uri stsUri, String targetServiceAudience, String authenticatorToken, HttpWebRequest request, TimeSpan timeout, EventTraceActivity eventTraceActivity, TimeSpan& expirationDuration)
   --- End of inner exception stack trace ---
   at Microsoft.Activities.Hosting.Security.OAuthS2SSecurityTokenServiceCredential.FetchAccessToken(Uri stsUri, String targetServiceAudience, String authenticatorToken, HttpWebRequest request, TimeSpan timeout, EventTraceActivity eventTraceActivity, TimeSpan& expirationDuration)
   at Microsoft.Activities.Hosting.Security.OAuthS2SSecurityTokenServiceCredential.GetAccessTokenFromTokenService(OAuthS2SPrincipal client, OAuthS2SPrincipal targetServiceAudience, HttpWebRequest originalRequest, EventTraceActivity eventTraceActivity, TimeSpan& expirationDuration)
   at Microsoft.Activities.Hosting.Security.OAuthS2SSecurityTokenServiceCredential.GetAuthorization(OAuthS2SAuthenticationChallenge[] bearerChallenges, HttpWebRequest request, EventTraceActivity eventTraceActivity)
   at Microsoft.Activities.Hosting.Security.OAuthS2SAuthenticationModule.AuthenticateInternal(String challenge, WebRequest request, OAuthS2SCredential credential, EventTraceActivity eventTraceActivity)
   at Microsoft.Activities.Hosting.Security.OAuthS2SAuthenticationModule.Authenticate(String challenge, WebRequest request, ICredentials credentials)
   at System.Net.AuthenticationManager.Authenticate(String challenge, WebRequest request, ICredentials credentials)
   at System.Net.AuthenticationState.AttemptAuthenticate(HttpWebRequest httpWebRequest, ICredentials authInfo)
   at System.Net.HttpWebRequest.CheckResubmitForAuth()
   at System.Net.HttpWebRequest.CheckResubmit(Exception& e, Boolean& disableUpload)
   at System.Net.HttpWebRequest.DoSubmitRequestProcessing(Exception& exception)
   at System.Net.HttpWebRequest.ProcessResponse()
   at System.Net.HttpWebRequest.SetResponse(CoreResponseData coreResponseData)
   --- End of inner exception stack trace ---
   at Microsoft.Workflow.Common.AsyncResult.End[TAsyncResult](IAsyncResult result)
   at Microsoft.Activities.Hosting.HostedHttpExtension.HttpRequestWorkItem.HttpRequestWorkItemAsyncResult.End(IAsyncResult result, Int32& responseCode)
   at Microsoft.Activities.Hosting.HostedHttpExtension.HttpRequestWorkItem.OnEndComplete(ScheduledWorkItemContext context, IAsyncResult result)
Nu opnieuw proberen 

 I created a new developer site collection and created the most basic app with a List and a Workflow, and in this app on this site collection I get the same problem, so it must be something with my account...

Searching for internet only shows me something with Timer Jobs on Sharepoint Server, however I have Sharepoint ONLINE.

Really no clue what to do with this, anyone an idea here?

Thanks in advance!