HTTP Error 500 after changing themes

[niggo04]

Hello.
I have a problem with setting up pico. After I copied all the files to my webserver everything works great. I see the sample page with the default theme.

Next step: install a theme. I've tried it with 'Freelancer' and 'Dimension'. After I copied the files I edit the config.yml and set the theme - and now I get a HTTP ERROR 500. And the error still remains when I undo the edit in config.yml and set the theme to default again...

What to do?

Best,
Nico