Thank you for your answer, Sean
Since 2.4 is a new release I report my conditions. It might help to get it as clean as possible.
2.3 ran (almost) perfect. But before I wanted to go about a strange behavior I wanted to upgrade. And that is the point when I got stuck and had to turn to this forum. It might be that the installation (Webinstaller) works fine. But if you upgrade you encounter problems. So I did a clean install on a machine that 2.3 was running fine.
after extensive trying I got Silverstripe to log. The logflie paths should look something like this: "/inetpub/log/ss.log" starting from the drive where PHP is located, so in most cases c:. "/log/" will not work. There's got to be a logfilename! Maybe a note in the config file would save time for others.
the Logfile states: " Error at \sapphire\thirdparty\Zend\Cache.php line 208: Uncaught Zend_Cache_Exception: cache_dir is not writable"
finding out with procmon I can see that PHP tries to access c:/windows/temp/
concluding ss tries to cache files in c:/windows/temp/
so how can I set the cache_dir ?