This may occur when you attempt to save a rule that has 51 or more products (or collections) configured. If this is the cause, reduce the number of products (or collections) to 50 or fewer and try saving again.
Server errors can be caused not only by issues with our app, but also by issues on Shopify's end. Please check the Shopify Status page to see if there are any known issues with Shopify. If this is the cause, you will need to wait until Shopify resolves the issue.
If the issue persists, please contact our support team.