We believe we've narrowed down and fixed the issue with Joomla 3.8.4. It's a combination of issues with updates to URL generation for SEF, the language filter plugin and a few other things. If you aren't using the Language Filter plugin, you should be able to disable it while leaving SEF URL's enabled and that should fix things for your as well.
If you need a fix ASAP, please PM me FTP credentials for your site and we'll gladly make the code changes necessary for you to test out. Otherwise, there will be a new release of JFBConnect very soon that should resolve this.
Joomla 3.8.4 is a security update, and we recommend moving to it, though the 4 security issues are rated as 'Low'.
www.joomla.org/announcements/release-new...a-3-8-4-release.html
Thank you,
Alex