if it's worth mentioning droplet name is mainsite.com and fqdn is set up properly. If it's helping i can post also the contents of hosts and hostname.

Another question would be if any other solution does not work, does it make sense to uninstall apache and to reinstall it. Is there any tutorial for this?

Thanks,
Vlad

"does it make sense to uninstall apache and to reinstall it. Is there any tutorial for this?"

probably not. if apache itself is working (websites being served) than it would be just more work

the problem, it seems, is in the conf files somewhere.

Here is a working conf file I have that I usually just copy for all other sites. The host this runs on has 7 sites that all run identical conf files.
Each site uses an SSL cert from StartSSL

<VirtualHost *:80>
ServerAdmin webmaster@domain.com

    DocumentRoot /var/www/domain.com

    <Directory /var/www/domain.com>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        Order allow,deny
        allow from all
                Require all granted
    </Directory>

    ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
    <Directory "/usr/lib/cgi-bin">
        AllowOverride All
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Order allow,deny
        Allow from all
    </Directory>

    ErrorLog ${APACHE_LOG_DIR}/error.log

    # Possible values include: debug, info, notice, warn, error, crit,
    # alert, emerg.
    LogLevel warn

    CustomLog ${APACHE_LOG_DIR}/access.log combined

    Alias /doc/ "/usr/share/doc/"
    <Directory "/usr/share/doc/">
        Options Indexes MultiViews FollowSymLinks
        AllowOverride All
        Order deny,allow
        Allow from all
    </Directory>
ServerAlias www.domain.com
ServerName domain.com

</VirtualHost>

<IfModule mod_ssl.c>
<VirtualHost *:443>
SSLEngine on

     ServerName domain.com
     ServerAlias www.domain.com

     SSLCertificateFile /etc/apache2/ssl/domain.com/domain.com.crt
     SSLCertificateKeyFile /etc/apache2/ssl/domain.com/domain.com.key
     SSLCertificateChainFile /etc/apache2/ssl/domain.com/sub.class1.server.ca.pem

     ServerAdmin webmaster@domain.com
     DocumentRoot /var/www/domain.com
     ErrorLog /var/www/domain.com/logs/error.log
     CustomLog /var/www/domain.com/logs/access.log combined
</VirtualHost>
</IfModule>

i am continuing with trial&error. I have unregistered all sites besides the 000-default.conf. At that moment nvh2.com and mainsite.com were pointing correctly to what 000-default.conf is indicating (/var/www/droplet_html).

I tried to register again only nvh2.com and it goes again wrongly to mainsite.com (even though mainsite.com is not registered). The issue is that mainsite.com has ssl configured so when I am writing http://nvh2.com in browser I am redirected to https://nvh2.com, which actually try to load https://mainsite.com.
I've tried to register again mainsite.com which is working correctly but now after unregistering both nvh2.com and mainsite.com I get redirected for both sites to the https version and I am getting a "Secure Connection Failed" error when in fact they should point again to 000-default.conf...

Secure Connection Failed
An error occurred during a connection to willuebersiedeln.at. SSL received a record that exceeded the maximum permissible length. (Error code: ssl_error_rx_record_too_long)
The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
Please contact the website owners to inform them of this problem. 

now I am trying to find a way to get back to that configuration were no sites are registered but due to DNS setup they are all pointing to 000-default.conf and also what is in 000-default.conf is served...

I've solved a part of the problem: browser history are hindering with http / https requests for visited sites: i.e. if you visited a site that has https browser will always serve https.
I have deleted browser history and cache and now with all sites unregistered and only 000-default.conf everything points out correctly. I will now load one by one the conf files and see if this will help.

And another: on mainsite.com a wordpress is installed. On nvh2.com were also copied the sources of wordpress and I was about to install it. Once I have deleted wordpress from nvh2.com directory this is not pointing anymore to mainsite.com.

Direction is clear I have to research how to install 2 wordpress sites one with https and one without on the same server using virtual name servers.