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.
A website on a shared hosting can consume all system resources and disrupt the performance of other websites. This issue is known as the “noisy neighbor problem”. Plesk Cgroups Manager is an extension that helps you address this issue by managing the consumption of the CPU, RAM, and disk read...
docs.plesk.com
They solved your issue already, let them know it's solved.