Internal Server Error or 'Data not Available' when Editing a Salesforce Page Layout

The following steps address the most common causes of this error in both Salesforce Classic and Lightning Experience.

Step 1: Clear Your Web Browser Cache and Cookies

Browser cache and cookie issues can cause web pages to perform abnormally. Clear your browser cache and cookies and retry editing the page layout

Step 2: Try Replicating the Error with the Legacy Page Layout Editor

Disable the Enhanced Page Layout Editor to test whether the issue is specific to it:
In Salesforce Classic: Go to Setup | Customize | User Interface.
In Lightning Experience: Click the Gear icon, then go to Setup | User Interface | User Interface.

1. Deselect Enable Enhanced Page Layout Editor.
2. Save.
3. Edit the affected page layout.

If the error no longer occurs, the Enhanced Page Layout Editor is the source of the issue.

Step 3: Check Your Managed Packages

Managed Packages have hidden integrations and dependencies. An expired Managed Package can cause unexpected behavior including this error.

• Ensure Managed Packages are in an Active status by reviewing your expired packages.
• If a package is still needed, a System Administrator should contact the package provider to extend the license.
• If the package is no longer needed, uninstall it.
• Ensure the user editing the page layout has a valid license for any relevant Managed Packages.