While working, sometimes, you may get an error generated by Visual Workflow, which may be too general, for example, We're sorry but a serious error occurred. Please don't close the Cloud Flow Designer window. Contact Salesforce Customer Support as soon as possible. Visual Workflow has a prebuilt debugger tool that allows you to debug your Flow from the Flow canvas itself. To open the debug window, press Ctrl + Shift + M (Windows) or command + Shift + M (Mac) while you open the Flow canvas. This will look like what is shown in the following screenshot:

To view more details about this kind of error, you can use the Visual Workflow inbuilt debugging tool. To get more information about an error, make sure that you select the Errors checkbox.