Hi Anne,
Try to find in which list the item is not found using the list id: 7e574d34-1012-4cee-94f0-92b74de48867
Then try to understand why the item you are processing is not in this list.
Maybe linked to a Workflow Task list.
Regards,
Christophe
Thank you, Christophe. We found that a Sharegate setting changed a workflow permissions action during migration, which disallowed certain permissions. Username was listed in Sharegate as the service account email address, it should have been programmed as the variable, see image: 
Still having trouble with the workflow not finding the list, but only intermittently. Still looking into that using Christophe's recommendation.
Anne Cipolla be aware of two things - first, Sharegate is changing list GUID during migration thus this value is different between your QA and PROD env.
And the second thing, really bizzare: I found many issues when working with lists/ libraries migrated by Sharegate. For ex. lazy approvals were not working for migrated task list and copying a file was not working for migrated library. After I created those apps from a scratch, all Nintex actions worked smoothly.
When I reported those issues to Sharegate they made some investigations following me up with the results. They suggested to do migration in to steps - first structure, second - contents.
In the end they said that since Sharegate 5.15.3 these issues should be fixed. Which version of Sharegate you are using?
I'm afraid I'm not the Sharegate user, just the SharePoint/Nintex user. Our Sharegate Admin is out until next week but I'll bring it up to him upon his return. Your comments about Sharegare behaviour would explain the issues we're experiencing, we'll be looking into that next week as well. Thank you so much for replying to this post!
You can try and create a new list then export->import the workflow to verify if it works for a fresh list.
You can also check if the item with ID=21 actually exists 
Maybe on the PROD list that item is not present
You can also try to reconfigure the action that is pointing to the list in question; sometimes just selecting the list in the new environment solves the issue. It's probably a query list action or something similar.
do you find any from the answers as the solution for your question? If so can you mark it as "Correct"?
Regards,
Tomasz
Hi Tomasz. We didn't know it at the time this post was initiated but the list item issue referenced here turned out to be part of a larger permissions-based problem. Working with Nintex Support, we found a workflow-based solution, post and solution can be found here: O365 Update Item Permissions.
Glad to hear that Anne Cipolla can you please mark any from the answers as "correct" (yes, this can be your answer either ).
Regards,
Tomasz
