Also ich seh auf anhieb nicht wo der Fehler sein soll.
Domain die nicht geht:
<Directory /var/www/domain.tld>
        Options FollowSymLinks
    AllowOverride None
    Order Deny,Allow
    Deny from all
</Directory>
<VirtualHost *:80>
      DocumentRoot /var/www/domain.tld/web
  
    ServerName domain.tld
    ServerAlias 
www.domain.tld
    ServerAdmin 
webmaster@domain.tld
    ErrorLog /var/log/ispconfig/httpd/domain.tld/error.log
    ErrorDocument 400 /error/400.html
    ErrorDocument 401 /error/401.html
    ErrorDocument 403 /error/403.html
    ErrorDocument 404 /error/404.html
    ErrorDocument 405 /error/405.html
    ErrorDocument 500 /error/500.html
    ErrorDocument 503 /error/503.html
    
    <Directory /var/www/domain.tld/web>
        Options FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    <Directory /var/www/clients/client1/web52/web>
        Options FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    # suexec enabled
    SuexecUserGroup web52 client1
    # Clear PHP settings of this website
    <FilesMatch "\.ph(p3?|tml)$">
        SetHandler None
    </FilesMatch>
    # php as fast-cgi enabled
    <IfModule mod_fcgid.c>
      # SocketPath /tmp/fcgid_sock/
      IdleTimeout 3600
      ProcessLifeTime 7200
      # MaxProcessCount 1000
      DefaultMinClassProcessCount 3
      DefaultMaxClassProcessCount 100
      IPCConnectTimeout 8
      IPCCommTimeout 360
      BusyTimeout 300
    </IfModule>
    <Directory /var/www/domain.tld/web>
        AddHandler fcgid-script .php .php3 .php4 .php5
        FCGIWrapper /var/www/php-fcgi-scripts/web52/.php-fcgi-starter .php
        Options +ExecCGI
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    <Directory /var/www/clients/client1/web52/web>
        AddHandler fcgid-script .php .php3 .php4 .php5
        FCGIWrapper /var/www/php-fcgi-scripts/web52/.php-fcgi-starter .php
        Options +ExecCGI
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    # add support for apache mpm_itk
    <IfModule mpm_itk_module>
      AssignUserId web52 client1
    </IfModule>
</VirtualHost>
Domain die funktioniert:
<Directory /var/www/domain.tld>
    AllowOverride None
    Order Deny,Allow
    Deny from all
</Directory>
<VirtualHost *:80>
    DocumentRoot /var/www/clients/client18/web31/web
    ServerName domain.tld
    ServerAlias 
www.domain.tld
    ServerAdmin 
webmaster@domain.tld
    ErrorLog /var/log/ispconfig/httpd/domain.tld/error.log
    ErrorDocument 400 /error/400.html
    ErrorDocument 401 /error/401.html
    ErrorDocument 403 /error/403.html
    ErrorDocument 404 /error/404.html
    ErrorDocument 405 /error/405.html
    ErrorDocument 500 /error/500.html
    ErrorDocument 503 /error/503.html
    
    <Directory /var/www/domain.tld/web>
        Options FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    <Directory /var/www/clients/client18/web31/web>
        Options FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
    # suphp enabled
    <Directory /var/www/clients/client18/web31/web>
        suPHP_Engine on
        # suPHP_UserGroup web31 client18
        AddHandler x-httpd-suphp .php .php3 .php4 .php5
        suPHP_AddHandler x-httpd-suphp
    </Directory>
    # add support for apache mpm_itk
    <IfModule mpm_itk_module>
      AssignUserId web31 client18
    </IfModule>
</VirtualHost>