WARNING [cache] Failed to save key

Question

I get the following warning whenever I do a SQL call to a MySQL database

07:36:46 WARNING   [cache] Failed to save key "%5B%5BC%5DAppBundle%5CEntity%5CBusinessUnit%24_maxid%5D%5B1%5D" (integer) 
["key" => "%5B%5BC%5DAppBundle%5CEntity%5CBusinessUnit%24_maxid%5D%5B1%5D","type" => "integer","exception" => ErrorException { …}] []

What is the cause of this warning?

How serious is it?

How can I prevent it?

I'm using a nginx server and the Symfony 3.3.5@stable framework.

Edit

This works fine in development and only gives these errors on the production server.

Answer 1

It is permission of the folder. On Ubuntu in the folder of project:

sudo chmod 700 -R var/