Hey @carinaa,
The usual cause of this issue is WPML creating a translated page before Pro or Cornerstone so the builder couldn’t generate its own way of translation. You can check those translated pages in the Translation Management page of WPML. I see your page in there.
I deleted the page generated by WPML (number 1 in the screenshot). The page is empty anyway.
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Regretfully, that didn’t fix the issue so the next thing I did was reset your permalinks. I did that by going to Settings > Permalinks and clicking the Save Changes button.
Now when I click the British flag, I got a different error in the console. It now returns 403 Forbidden for the WPML URL (see secure note) rather than a generic 500 Internal Server error. It got specific.
###Before:
###After
The issue now is either your web host or something in your setup is blocking the WPML URL. Please check with your web host why the URL is being blocked. It could be some kind of security feature blocking it.
Thanks.