WordPress database error

Hello,

I get this error randomly when accessing several articles in a row:

http://imageshack.com/a/img745/7180/RFn61A.png
http://imageshack.com/a/img905/1525/MPu8Qp.png
(they are both the same error. I couldn’t take one print screen or copy the entire thing).

Any idea what can it be?

Thanks.

https://wordpress.org/plugins/wordpress-23-related-posts-plugin/