While there are likely a number of causes to this problem, I’m writing in regard to a specific scenario in which a SharePoint Designer 2010 workflow automatically replaces item-level permissions for a document based on the contents of a people column. It’s possible this solution will apply to a different scenario, however, so read on.
The people column’s settings are multiple selection, people AND groups as can be seen here. This is absolutely fine (multiple selection, mixing of permission groups with individuals). You may just note the warning in red in the screenshot below:
“Earlier versions of client programs might not support a Person or Group field that allows multiple selections. Adding this field might block those programs from saving documents to this library.”
In your workflow, just make sure you’re returning your people column “As String” and not any of the semi-colon delimited options. That’s all there is to it. Publish the revision and run the workflow again (and you may need/wish to go in and end any workflows that errored out but are still “running”). Best of luck!