If the org has more than 1000 active classes, then /apex/classDiagramCreator returns an error: "Collection size XXXX exceeds maximum size of 1,000." Could be due to apex:repeat only supporting collections of up to 1000 items: https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_repeat.htm Similar to https://github.com/afawcett/apex-umlcanvas/issues/8