In ArcGIS Online, unable to export a hosted feature layer as a file geodatabase (FGDB) if it contains related tables, and returns the error message, “There was an error."
The issue is not reproducible in the current version of ArcGIS Online. A hosted feature layer containing related tables can be exported as a file geodatabase with no issues. If the export process fails, contact Esri Support Service to troubleshoot the issue and identify the cause of the error.