Thank you for your help!
 
	I actually didn't have to install the PW first. 
	I was able to use the old version and only had to enter the new access data for the database.
 
	The error was caused by a few cache files (*.cache). 
	I deleted them and the site works!
 
	I still have one question: 
	How do I get into the backend? /processwire/ does not work. 
	Could it be that the backend login has been renamed? Can I find out? I have already looked in the database, but found nothing.