I'm assuming you're running Joomla 3.8 (or 3.8.1). There's many reports of this happening with *any* Joomla extension and there aren't fixes we can implement on our end to get around this. It seems like a Joomla corruption issue either with the database or filesystem after the Joomla 3.8 upgrade. For more details, see:
github.com/joomla/joomla-cms/issues/18025
github.com/joomla/joomla-cms/issues/18020
github.com/joomla/joomla-cms/issues/18047
github.com/joomla/joomla-cms/issues/18098
There are a lot of details in each of those threads about some possible work arounds to find the issue. I'd highly suggest restoring your site from a backup from before you upgraded JFBConnect though and then simply re-installing again. Most of the threads above say that will fix the issue as the chance of an upgrade issue is slim each time you do it.
If you don't have a backup, you can send us a Private Message with admin credentials for your site, but there's only so much we'd be able to do. Admin creds don't allow us to edit the filesystem or make changes to the database. If there is something we can do for JFBConnect, we'll gladly try, but if it's a case of Joomla itself having a corruption problems, there's little we'd be able to do in that case.
Thanks,
Alex