Actually I hadn't, however after installing and trying to set up the newer version, the following appeared on the front end: (screenshot in the attachment)

Hello,
1.) Disable all other Cache Tools or Compression tool and clean the cache > STRG+F5
2.) Install the Astroid 2.5.11
www.joomlaplates.de/login.html
3.) Disable the Contact Module

I did everything, however the error remains.

Hello, ok guessing is useless.
Give me Joomla admin access via contact.
www.joomlaplates.com/contact-us