Issue is now resolved. Thanks.
To help others if you experience a similar issue here is what we tried and what fixed it. As per suggestions above and via emails from staff, we;
- moved the topic to another forum
- tried default theme and other themes
- turned off rocket loader on Cloudflare
- checked topic.phtml files (we had several) as per suggestion: "a few comments/articles outside of the #elPostFeed form. Can you compare this template with the default and fix any discrepancies to see if that fixes the issue?"
Afterall, the problem was with a poorly formatted message from a member of our forum which broke the quote function. Editing/saving it fixed the issue.
