The problem is already solved. I tried to make some implementations for showing the error log. Then I re-downloaded Enfold via ThemeForest and now installation and activation worked.
I deleted the enfold theme via FTP. Now the website works again with the standard WordPress theme.
Could you have another look at the page, if you can do anything about it?
Okay, tried to set up the error log. Then redownloaded Enfold from ThemeForest and now it works.
No Plugins installed at all. WordPress login details are already above. In FTP, there are no error log files available. FPT login details also already in private.