How to fix Failed to fetch an access token from the token service on a SharePoint Online workflow

Question

We have SharePoint Visual Studio workflows that run every day and have been for months. However today on three different client sites, the workflows are getting this errors. Last time we got this error, we were able to go to the workflow's permission's page and click trust to get it working again. We also deactivated and reactivated Workflows can use app permissions under Site Settings -> Site Features. and erminated the workflow and restarted the workflow and received the same error.

https://client-bd9589250b78de.sharepoint.com/sites/mysite/workflow_name/_api/sp.utilities.utility.ResolvePrincipalInCurrentcontext?input=%27i%3A0%23.f%7Cmembership%7Cuser_company.com%23ext%23%40client.com%27&scopes=%2715%27&sources=%2715%27&inputIsEmailOnly=%27false%27&addToUserInfoList=%27False%27&matchUserInfoList=%27true%27


System.Net.WebException: The request was aborted: The request was canceled. ---> System.InvalidOperationException: Failed to fetch an access token from the token service. For details, see the inner exception. ---> System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   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.AuthenticationManagerDefault.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.OnEndComplete(ScheduledWorkItemContext context, IAsyncResult result)