I use AddThis Share Buttons on my site (this add-on was installed in ready-made WP theme). I have four social buttons: FB, Twitter, VK and LinkedIn.
The problem is that when you try to share the post, the page gets error 400. In different browsers different buttons doesn't work: Chrome 46 — twitter, vk, linkedin; FF 42 — vk, linkedin; IE11 — all buttons work (:D)
I think it's because of too long request - but I can be wrong. How to struggle it? Maybe someone had similar problems?
If the addon was installed by default in the theme, they may have used the addthis API. In that case, error 400 means a missing/invalid parameter or an exceeded rate limit for that particular API call.
For more details have a look at http://support.addthis.com/customer/portal/articles/381264-addthis-analytics-api
Additionally, if you can post your site URL, it will help us to find the problem causing api call url