That is interesting that it's the Language Filter. I'm familiar with FlexiContent, although it's been a couple of years since I've used it. It does extend Joomla content in the database, but it uses it's own URLs and routing to display that content, which means it's very different from Joomla content in the ways that matter for the issue you're having.
Looking at the page's HTML, even with JCH enabled, if you search for "jfbc.return_url", you'll see a long line like:
jfbc.return_url='L3NvdW5kLWVmZmVjdHMvaW50ZXJmYWNlLXVpLzE3NDIzLXNpbXBsZS1wb3Atc291bmQ=
That long string of garbage is the encoding for the current URL that JFBConnect uses to redirect the user after login. To decode it, you can paste it into
www.base64decode.org/
and it will show the SEF URL. For example, above, the resulting url is the one you posted originally:
/sound-effects/interface-ui/17423-simple-pop-sound
So, the URL we're creating looks correct. Why that would be breaking isn't something.
Now, I just tested at the URL above and both after registering for the first time and then logging in I was left on the same page.
Is the Language Switcher Plugin currently disabled? If it's enabled, we'll need to understand better how to recreate the issue as it seems to be working properly now.
Thanks,
Alex