Since a couple of months I have encountered the User cannot be found error a few times when accessing the List Settings or Library Settings.
In my case I encountered this error in the following scenarios:
- After switching the web application to Claims-based authentication;
- After migrating some sites to another SharePoint farm.
The solution is to republish all existing workflows (also the default ones) through SharePoint Designer 2010. The problem arises due of the fact that the workflows were published by a user who is not known by SharePoint.
In the Claims-based authentication scenario the user did not have the claims-based prefix before its username (ex: i:0#.w‘domain\username).
In the Migration scenario the sites were restored in another AD domain.
To republish the workflows you can follow the next steps:
Open your site in SharePoint Designer 2010;
Click on the Workflows link from the Site Objects menu;
Click on the workflow name, this will open your workflow;
Click on the Publish button;
Do step 3 and 4 for all the workflows.
Go back to your List or Library Settings and the error should be gone.