I use it every day. I can confirm it works well with 7.x. I haven’t tried it on 8 yet, but I would strongly suspect it doesn’t work as I know the API url for 8 is slightly different. The plugin can’t work for both 7 and 8 without changing the API URL is what I’d predict.
Make sure you use the SugarCRM 6.x version for the Mautic plugin NOT the 7.x version.
