When a Contacts email address is updated AND that contact has a related Subscription, the "Print Email Receipt" field on the Subscription is updated with the new Contacts Email address.  This is so workflows, in particular the annual renewal are sent to the correct/updated email address.

What can happen is if a  Flow/Process Builder or Workflow has been added that is triggered by a update on the Subscription record and that updates the related Contact, then this creates a loop between the Contact and Subscription.  That is the root cause of this error and it would be an endless cycle if not prevented.

You need to either disable  the Flow/Process Builder/Workflow or update the entry Criteria so that it does not always run or does not run if the email address is changed.  

NOTE, our standard support does not custom Flow/Process Builder/Workflows.  You will need Premium Support if you need further assistance.