We have several Tabs overridden by Visualforce pages. Ohter Visualforce pages are reached via a Link.

Until Summer 18 as default all these pages were rendered with Header and Sidebar.

Now starting today, the Headers and Sidebars are missing. I've already tried to set some attributes explicitly on the apex:page

<apex:page standardController="MyCustomObject__c"extensions="MyApexExtension" showHeader="true" sidebar="true" standardStylesheets="true">

Still all the pages are rendered without Header and Sidebar.

Is there any way to get the pages redered with header and sidebar again?