Topic
Updating an existing Exchange Online instance fails on the OAuth Token step with the following error:
Error issuing app only token.

You will also see this error message in the logs around the same timestamp:

"Error","General","1","GeneralErrorMessage","SourceCode.Security.OAuth.Extensions.MicrosoftOnlineAppOnly.IssueAppOnlyToken","1 One or more errors occurred.   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)   at SourceCode.Security.OAuth.Extensions.MicrosoftOnlineAppOnly.IssueAppOnlyToken(OAuthResource oAuthResource, String resourceAudience)    InnerException: An internal server error has occurred. Retry the request.

Instructions 
Please perform the steps below during a scheduled downtime or after office hours to avoid any form of disruption the K2 service restart may cause.

1. Restart the K2 service.
2. Clear the browser cache on the machine where you are running the wizard on.
3. Re-open the browser.
4. Update the Exchange Online instance