I know you don't like to upgrade from 1GB RAM to 2G for just 1 website or 1 extra plugin but it's advised to upgrade if your website gets more visitors within small period of time.
Another workaround is to use Cgroup to put hard memory limit on 1 Domain/Subscription to not affect Plesk and case that error.
summary
docs.plesk.com
They solved your issue already, let them know it's solved.