UPDATE: Gmail's team are aware of this issue. It has been reported by a few other companies. Google recently emailed Sortd to say that they are currently looking at a fix.
With the recent changes to the New Gmail interface, there is a known bug preventing a small percentage of emails from opening the first time you click on an email (particularly emails in sent items). You may see the message “The conversation you requested no longer exists” however the conversation is certainly still available and accessible from Gmail mode.
Sortd should open the email without a problem but it may take a second long for the email to load due to the way we are querying the email from Gmail.
We would appreciate your patience while we address this issue.