I'm running a WordPress website with a custom theme that's having some problems lately. After clicking a shared link on Facebook that points to my website, I sometimes get an character encoding error. I checked everything in the wp-config file and ran a database optimization + repair, but I still get notices from people that the page doesn't load properly. This only happens when the link is clicked from Facebook and the URL has some Facebook query string.

I saw this question, but this seems to be about the Open Graph posts description only.

Does anybody have any idea what is the cause of this problem, and how to fix it?

Thanks a lot in advance! :)