Taken from a Plesk support article that solves another issue, but probably the same solution can apply to your case:
Open /etc/sw-cp-server/config file using a text editor
Adjust timeout limits for FastCGI, for example:
fastcgi_read_timeout 1200
fastcgi_send_timeout 1200
Open /usr/local/psa/admin/conf/php.ini file using a text editor.
Adjust timeout limits for PHP, for example:
max_execution_time = 1200
Restart sw-engine and sw-cp-server services:
# service sw-engine restart
# service sw-cp-server restart
If the issue still persists increase timeout limits even greater.