Sending emails to the same address via Journey

Emails configured in Journey Builder are Triggered Sends, which you can find in the Interactions tab in SFMC.

You can pause the trigger corresponding to your email and download the queued subscribers or clear the queue. You can also pause and restart the trigger definition in case one email is holding up the queue.

I'm guessing there's some issue with the data coming over in the payload that isn't handled in the email scripting.

Edit

I'd revise your AMPScript to make it more fault-tolerant and to include the CustomerID which is required in the DE you're updating:

%%[

var @eid, @js, @cid

set @eid = AttributeValue("EventInstanceId")
set @cid = AttributeValue("CustomerId")

if empty(@cid) then
  set @cid = "0"
endif

set @js = now()

UpsertDE("Account Updates", 1, "EventInstanceId", @eid , "CustomerID", @cid, "Journey Status", @js) 

]%%

It's a good practice to code your email assuming that any of the incoming values can be null. The AttributeValue() function will return an empty string if the personalization string is undefined.

I just figured out this seems to be another feature/setting that isn't activated by default and has to be requested as a support case. So even if you see this feature in Email Studio when configuring triggered sends, you need separate activation for use within Journey Builder.

Error Threshold

A configurable limit that stops a send job when a designated number of errors occur.
This option helps prevent a single error from stopping the entire job. Contact your Salesforce Marketing Cloud relationship manager for more information.

(Source: Configure the Send Email Activity)

If the feature is activated there is a global threshold setting defined by support, that can be activated or deactivated in the send email activity via a checkbox. To be more flexible a user defined threshold on a per activity basis would make sense. Therefore I posted this on IdeaExchange. If you like the idea, please vote for it: User defineable Error Threshold in Journey Builder send email activity.