HELP - site crashed after installing JFBConnect 1.5 on 1.7 website

Markus,
The problem is likely the JFBCSystem plugin. I'm assuming it crashed after you enabled the JFBCSystem plugin. If so, you'll need to be able to access the database and disable that plugin by running the following query:
Once done and you can (hopefully) access the admin area, uninstall the JFBConnect component, JFBConnect authentication, user, and system plugins and the SCLogin module. Then, reinstall the correct Joomla 1.7 version.

Let us know if you run into any other issues or need more help!
Alex

Oh yeah, make sure you update the XYZ in that query above to your proper database prefix.