[Error] pdfforge.Mail.OutlookClient.ShowEmailClient: Error while starting Outlook System.Runtime.InteropServices.COMException (0x80004023)

Hi, We are running PDF Creator 5.0.3, I have recently upgraded most of our users to Outlook 2021 and a select few users are having issues with the send pdf function (error logged above) I have had a good rummage on here to find a solution and tried comparing the settings (both outlook and PDF creator) but no luck so far. Can anyone help?