I have found the solution on my own (and I’m astonished, that nobody supports this plug-in).
I used PHP, version 5.3:
That’s the reason, why I couldn’t activate the plug-in.
Change to a newer version of PHP. For example 5.6 will work.
You must check, which versions are supported by Your webserver system.
If You have no panel (like PLESK), where You can choose the right PHP version, you could also add to the .htaccess one of the following lines
AddHandler php70-cgi .php <- too experimental, don’t use
AddHandler php56-cgi .php <- my choice, which worked
AddHandler php55-cgi .php
AddHandler php54-cgi .php
AddHandler php53-cgi .php <- my former PHP version
AddHandler php52-cgi .php
AddHandler php5-cgi .php
