So, we have a ton of share links all over our various domains at work. And much of our domain structure is based on users have subdomains (example: JohnCongdon.herbalhub.com).
All of a sudden, the sharer.php seemed to stop working. Some browsers stayed white, Chrome showed a grey error message with a 500: internal server error.
It took me a long time to track down the problem. I tried so many different things, but in the end, it was the CASE of the subdomain. Changing it to johncongdon.herbalhub.com solved the issue. So strange, undocumented, and frustrating.
I hope this helps someone down the road.