How to resolve error message EmailHTMLBody: You can't change the standard controller object EmailMessage__c displayed while installing/upgrading e2a?

While a user is installing or upgrading to a newer release of e2a and receive the error message This app can't be upgraded, EmailHTMLBody: You can't change the standard controller object EmailMessage__c because this page is referenced in the page layout e2a Email Layout, which halts the install or the upgrade.

To resolve the issue the user needs to remove the Html Body Visualforce page form the e2a emails object layout and then try to re-install the release. 

Steps to remove the Html Body Visualforce page from e2a Email Layout

1. From Setup,
   1. Lightning: Go to Object Manager | e2a email | Page Layouts
   2. Classic: Enter Object in the Quick Find box, go to e2a email | Page Layouts 
2. Edit the available page layout named e2a Email Layout
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3. Once in Edit mode, Select Visualforce Page option from the menu available on the top-left
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4. Locate the Html Body Visualforce Page on the e2a email layout and remove by clicking on the   sign on the component
5. Once the Html Body Visualforce Page is removed from the e2a email layout, go ahead and click on Save button on the top-left of the page 

Once the Visualforce page is removed, go ahead and install/upgrade the release again and it will be installed as expected. 

Please contact us at support@ortooapps.com for any questions.