This error can happen where there is an orphaned report or folder in the database, which is one that is in a folder that doesn't exist. The resolution is to move that report or folder into a folder that does exist.
If you are Access hosted, we will identify the orphaned records and ask you which folder you want them put in to. Please raise a new case online and reference the title of this article.
If you are self-hosted, we will provide you with SQL to diagnose and then resolve the issue.