One client mentioned that previews of their articles do not work. Turns out that LinkedIn has their own debugging tool for such purpose: https://www.linkedin.com/post-inspector

It presented a 400 error that related to an nginx error: 414 Request-URI Too Large

The solution was to expand the relevant buffers:

large_client_header_buffers 4 3k;

That was enough to solve it for this particular case.

Facebook has a similar tool: https://developers.facebook.com/tools/debug/