No CSS visible at all!

I installed Elgg for the first time, following the installation instructions step by step.
After the installation everything ran very well for some time and I started to play around a bit.
I activated the option "Compromise CSS and JavaScript files" and also activated one of the included plugins in the settings - only to find out that I have a working Elgg page - but no CSS anymore.
I deleted all Elgg files and the /data folder, rebuilt the database and reinstalled Elgg, but the pages remained "unstyled".
Thanks for your help write if you need additional information and where I can find it,