Hello - I am struggling with locating the cause of an error that some of my users experiences some of the time when attempting to save data in a Nintex - classic form in SP2013 on-premise. The error references a concurrent user (message text below):

"Sorry, something went wrong: Save Conflict - Your changes conflict with those made concurrently by another user. If you want your changes to be applied, click Back to your Web browser, refresh the page, and resubmit your changes."

I have interviewed the end users, reviewed workflow and version history logs.

*There is only one end user in the form at the time of the error.

*There is not a workflow running on the form while they are in it. The workflows take a minute to run. All save buttons are "Save and Submit/Close" so the end user is getting the error when they are exiting the form.

*I've added a pause action on the modified workflow for 1 minute hoping that the time the form saves and the workflow runs would potentially prevent this error. That hasn't helped.

*They are only opening the form in one browser.

*Time out doesn't seem to be an issue as the error. I've tested by leaving a form open for much longer than my end users and don't get the error.

*It's happening across multiple browsers.

*It's intermittent. For example, one user submitted updates to four items on the same day. Three of the items submitted successfully, one of the items produced the error.

What could be other potential causes of concurrent user error message? Any suggestions on where to look or how to trouble-shoot? I have not been able to recreate the error on my own based on description of the issue (one user, one browser tab open, one save/close). 

Thanks in advance.