Hey Ask,
from the error message I suspect that it might not be connected with the slider, but with the php code of the array that constructs the WP_Query object for the one-page layout. Please download this archive, unzip it, and upload the resulting files into the theme folder (/wp-content/themes/Kanso/) via ftp, replacing the existing files when prompted. Let me know if the error is still there after refreshing the website!
Sincerely,
Andrii / Satori Studio