Validation failed: Invalid input in "Redis socket"

I always fails to save configuration settings and get this warning:
Validation failed: Invalid input in "Redis socket"
I installed GRAV 1.5.6 Core + Admin Plugin in: Subfolder not rooted http://prikitiew-adventure.com/blog/, using shared host, Apache 2.4.37, PHP 5.6.39, and adding just one line in htaccess: RewriteBase /blog/

is there something I did wrong? thanks for help