In [1], it seems that for very large reports, where the preview output may be causing issues, the recommended workaround was to configure com.knime.server.webportal.disable_report_preview=false as per [2], so that the preview will be disabled but the output can still be downloaded.
I do not find any registered defects regarding this issue in our issue tracking database.
[1] Report Designer error: Object doesn't exist
[2] KNIME WebPortal Administration Guide