BR,
First off, we just released v4.3.4 of JFBConnect yesterday which fixes the downgrade/upgrade issues that you ran into before.
For the issue you're currently having, please go into the Configuration -> Misc tab and in the Experimental settings area, enable the "Strip tags from head" setting, or whatever it's called. Test to see if that fixes your problem and let us know how it goes.
Please, please, please, test many pages on your site to make sure everything works as expected. That includes pages that were working fine before. The experimental setting can cause issues on all pages of your site. In the past, before some recent changes, it was making some pages completely blank; which is why it's disabled by default. We're hoping this release doesn't cause the issues we've seen in the past, but we need user-confirmation and testing to verify.
Thanks,
Alex