When submitting a form in Umbraco 8, I get the below error in the log, and the mail is not send. I've tried reinstalling Forms but this hasn't had any effect yet... Any suggestions?

{"@t":"2019-04-19T15:44:58.4202270Z","@mt":"There was a problem sending an email to {Email} from Workflow for Form {FormName} with id {FormId} for Record with unique id {RecordId}","@l":"Error","@x":"System.NullReferenceException: Object reference not set to an instance of an object.\r\n at Umbraco.Forms.Core.Services.XmlService.ToXml(RecordField recordField, XmlDocument xmlDocument)\r\n at Umbraco.Forms.Core.Services.XmlService.ToXml(Record record, XmlDocument xd)\r\n at Umbraco.Forms.Core.Providers.WorkflowTypes.SendEmail.Execute(Record record, RecordEventArgs e)","Email":"[email protected]","FormName":"Contact form","FormId":"09cc9ab8-93ef-45b3-8ec6-63c4939f4df6","RecordId":"879ebe4d-7876-4894-93c5-f506d60cbff7","SourceContext":"Umbraco.Forms.Core.Providers.WorkflowTypes.SendEmail","ProcessId":1688,"ProcessName":"w3wp","ThreadId":53,"AppDomainId":9,"AppDomainAppId":"LMW3SVC2ROOT","MachineName":"TRANSIP-VPS","Log4NetLevel":"ERROR","HttpSessionId":"b1r3yco0ccwmahofenpanqub","HttpRequestNumber":10,"HttpRequestId":"33b4f0fd-99ca-4248-bdae-0f1688e7eea6"}