Salesforce has released the Custom Setting Critical Update to all Sandbox orgs. With this update, there is a known issue where MyInsights reports, when viewed online, might make a call to Salesforce to get a field from Veeva Settings which causes the MyInsights report to fail. This issue is affecting all Sandbox orgs or Production orgs where the Critical Update was manually applied.
Salesforce Custom Settings Critical Update - Known Issue
To resolve this issue admins can give individual user profiles full access to Custom Settings or the critical update can be reversed manually by disabling that security setting in Setup -> Data Management -> Schema Settings -> Restrict access to custom settings.