Internal server error or 'Data not Available' when editing a page layout

Clear your web browser cache and cookies

Issues with the browser can cause web pages to perform abnormally when the cache and cookies are due for a refresh. 

Try replicating the error in the old page layout

1. 
In Salesforce Classic: Go to Setup | Customize | User Interface. 
In Lightning Experience: Click the Gear icon then go to Setup | User Interface | User Interface
2. Deselect Enable Enhanced Page Layout Editor
3. Save
4. Edit the affected page layout.

Check your Managed Package

• Managed Packages have many hidden integration and dependencies, so the expiration of these packages can cause unexpected problems.
• Be sure Managed Packages are in an 'Active' status. Do this by reviewing your expired packages to determine if they are still needed. If so, a System Administrator should contact the package's provider and have the license extended so that it's back in an 'Active' state. Otherwise, it should be uninstalled.
• Ensure the User making the edits has a license to the Package. Ask the System Administrator if uncertain.