Hello Markus,

it looks like the Jyaml Framework installed on Joomla is using some custom JHtml classes to override the default one of Joomla and when another template is used those classes are not loaded, thus the error happening.

I don't know how the Jyaml framework works, if it's based on library or plugin, if you want me to have a look at the backend please contact me at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

Probably a good choice would be to contact the Jyaml developers to ask some more informations.

John