User having role "UI_Builder_admin" is not able to edit the landing page contents.
1) Log into any Z-booted/upgrade instance with a user having "UI_Builder_admin" role
2) Navigate to landing pages
3) Open any of the unprotected landing pages
4) Click on the option "Open in UI_builder"
The page is getting opened in UI Builder, but user is not able to add or edit widget content.
This problem has been fixed. If you are able to upgrade, review the Fixed In section to determine the latest version with a permanent fix your instance can be upgraded to.
As a workaround, user can login as admin in order to edit the landing pages.