Issue
The following error is thrown when your workflow is trying to send an email with Exchange Online:
SourceCode.Workflow.Shapes.Execution.Mail.EWS: Exchange error code: ErrorTooManyObjectsOpened Exchange error: Microsoft.Exchange.WebServices.Data.ServiceResponseException: Too many concurrent connections opened., Cannot open mailbox.
Resolution
The error message suggest K2 may have hit some sort of throttling limit set by Microsoft for Exchange Online.
To put it simply, Microsoft limits how many connections you can make concurrently to Exchange Online, and limits how many emails you can send out concurrently. This is not something that can be fixed from K2 as this is a limit that is determined by Microsoft. You need to take note of this to avoid hitting a similar error again in the future.
You can retry the error after a while and K2 should be able to resend your email.