RoundCube not showing HTML email in preview

I think you know the problem in following somebody’s ‘Howto’ install a combination of different packages to make it work together. I did it the other day to set up a shiny new email solution. So I installed Postfix, Devecot, Spamassassin and some other more or less useful applications, gismos and plugins. Fine. After changing the configuration files it started to work. Hours later I recognized that my eMail client RoundCube did not show any html email content in the preview. I googled for a solution and 2 days later I finally found the problem. You need to install the php-xml package for your PHP version. That did the trick eventually. Don’t forget to restart your Apache server and to clear your browser cache.
I saw a lot people out there having that problem but only one forum came up with this solution.