Add Links Not Working

I just have a couple of test members at the moment but neither of these regular user accounts can add links.

Clicking preview works fine but on clicking ‘Add Link’ it just removes the post to show original page.

Interestingly, clicking cancel also renders the preview and clicking ‘add post’ also removes the post.

As admin I can post links just fine.

BTW adding video and uploading images works fine for regular users too.

I have tried disabling all the current plugins and swapped out the theme many times. Currently using default WP twenty fifteen theme.

Any idea about this?

Staging site link
https://84c.40c.myftpupload.com/

https://www.ads-software.com/plugins/buddypress-activity-plus/