We had a frustrating and esoteric error when trying to save the Event Monitoring Analytics app in any sandbox. 

common.request.UserInfo$InvalidUserInfoParameterException: Invalid org id: 00D000000000Xx0 or user id.

This is the appropriate knowledge article.

https://help.salesforce.com/articleView?id=000233515&language=en_US&type=1 

Frustratingly, the article does not indicate the error message and also doesn't include that it can happen when you save an app and not just when enabling analytics. So, we wasted quite a lot of time and multiple case escalations before getting an agent who was able to find the solution. I'm posting this so that others can quickly find the solution. 

To be specific, this was our solution

If your environment has restricted email domains, you will have the following option under Setup | Manage Users | Allowed Email Domains:

• Please add "[18-character Org ID].com" to the Allowed Email Domain list. You can generate your 18-character Org ID with the Record ID Converter bookmarklet.  (example: 00D00000000AaaaHHH.com)
• Note: The 18-character ID is overwritten with the Production 18-digit ID when a Sandbox is refreshed. You must add a unique 18-character ID for the sandbox again after refresh.