TYPO3 - after Upgrade from v10 to 11 Site shows : Please select a page template in page properties

Question

after my upgrade from v10 to v11 my TYPO3 page https://testing.nellbreuningschule.de/home only shows

Please select a page template in page properties.

Page Template is set in the Page Configuration but there is still no output. Installation is a composer installation and has the latest updates.

Does anybody has a clue for me?

THe Following Extensions are installed:

 TYPO3 Admin Panel  adminpanel  v11.5.9 stable  System  
 TYPO3 Backend  backend v11.5.9 stable  System  
 Tools>Log  belog   v11.5.9 stable  System  
 Backend User Administration    beuser  v11.5.9 stable  System  
 TYPO3 Blog Extension   blog    dev-master  stable  Local   
 Cookieman  cookieman   2.9.17  stable  Local   
 TYPO3 Core core    v11.5.9 stable  System  
 Dashboard  dashboard   v11.5.9 stable  System  
 Example Theme  demotemplate    11.2.9  stable  Local   
 Extbase Framework for Extensions   extbase v11.5.9 stable  System  
 Extension Manager  extensionmanager    v11.5.9 stable  System  
 Frontend Login for Website Users   felogin v11.5.9 stable  System  
 File>List  filelist    v11.5.9 stable  System  
 Advanced file metadata filemetadata    v11.5.9 stable  System  
 Fluid Templating Engine    fluid   v11.5.9 stable  System  
 Fluid Styled Content   fluid_styled_content    v11.5.9 stable  System  
 Flux: Fluid Integration    flux    dev-development beta    Local   
 Form   form    v11.5.9 stable  System  
 TYPO3 Frontend library frontend    v11.5.9 stable  System  
 Google Maps API Extbase    go_maps_ext dev-master  stable  Local   
 Import/Export  impexp  v11.5.9 stable  System  
 Indexed Search Engine  indexed_search  v11.5.9 stable  System  
 Web>Info   info    v11.5.9 stable  System  
 System>Install install v11.5.9 stable  System  
 System > Configuration + DB Check  lowlevel    v11.5.9 stable  System  
 News system    news    9.3.1   stable  Local   
 Web>List   recordlist  v11.5.9 stable  System  
 Redirects  redirects   v11.5.9 stable  System  
 System Reports reports v11.5.9 stable  System  
 CKEditor Rich Text Editor  rte_ckeditor    v11.5.9 stable  System  
 Scheduler  scheduler   v11.5.9 stable  System  
 SEO    seo v11.5.9 stable  System  
 User>User Settings setup   v11.5.9 stable  System  
 Internal notes sys_note    v11.5.9 stable  System  
 Editor with syntax highlighting    t3editor    v11.5.9 stable  System  
 Template Maker templatemaker   1.3.0   stable  Local   
 Web>Template   tstemplate  v11.5.9 stable  System  
 VHS: Fluid ViewHelpers vhs dev-development beta    Local   
 Web>View   viewpage    v11.5.9 stable  System  
 SASS compiler for TYPO3    ws_scss dev-master  stable  Local   
 WS Slider - Universal slider extension ws_slider   dev-master  stable  Local   
 TYPO3 Bootstrap Template   ws_t3bootstrap  11.4.0  stable  Local

Thank you in advance.