How to setup virtual hosts on local server (ubuntu, xampp)?

Question

Apache HTTP Server

Hello everyone, for half a day I can’t figure out what’s going on
with httpd-vhosts.conf:

<VirtualHost *:80>
    DocumentRoot "/opt/lampp/htdocs"
    ServerName localhost
    <Directory "/opt/lampp/htdocs>
         AllowOverride All
         Require all granted
    </Directory>
</VirtualHost>
<VirtualHost *:80>
    DocumentRoot "/opt/lampp/htdocs/testsite"
    ServerName testsite
    LogLevel debug
    <Directory "/opt/lampp/htdocs/testsite>
         AllowOverride All
         Require all granted
    </Directory>
</VirtualHost>

localhost works fine, but testsite gives 500 error.
What is written at the same time in error.log does not fit into the question (more than 10,000 characters), the meaning is there in a cyclic redirect, as I understand it.
What to do, where to look?