If hosted, please contact support and reference this Knowledgebase article.

If self-hosted, please redeploy reports for the environment. Afterwards, recycle the application pool (which should be done after hours as it will disconnect users working in CRM). After the app pool has been recycled, log into CRM > Administration > Application > Catalog browser > Load all reports > Unmark the "Only load reports that have never been loaded" checkbox > Select all > Start.