I wanted to post this as I was banging my head against a brick wall for hours recently with this issue. Full credit goes to this stack overflow thread that described pretty much exactly what I was experiencing:
However, my situation was a little different so thought I would post this in case it helps someone else in future!
SharePoint 2010 environment using a standard SharePoint Designer workflow. Straight-forward workflow that runs once a list item is created, creates a document set in a separate library, then updates the list item with a link to the document set (plus some other item metadata). All this is wrapped up in an impersonation step that uses a site collection admin user account.
Why an impersonation step?
An impersonation step is being used because when you create a new list item using a workflow, that new item is created by the System Account. So on any new item created by a workflow, “Start workflow on create” will not work because system account is not allowed to start a workflow.
Intermittently the workflow would fail on creating the document set. The workflow status showed “The workflow could not create the list item. Make sure the list exists and the user has permissions to add items to the list.”. The outcome is “Unknown Error”.
After speaking with users this didn’t seem to affect everyone all of the time. It only affected some people occasionally but it was affecting one user way more than others.
This user was creating the list item in exactly the same way as others in the team. I overserved the user creating the list item and couldn’t see any problems with how it was being done. Yet each time we conducted a test copying field information from an existing item the workflow continued to error.
In my case, when I started to look at the list items where the workflow had failed, I noticed that the Name field for each list item contained a trailing space at the end of the text. I asked the user who this issue consistently affected to try creating new list items several times, copying the Name as normal but this time removing the trailing space and we no longer had issues!