@
taja - drat, I thought of suggesting you copy it first as a backup
Tip : whenever you're advised to 'edit' a file (whether it be for calibre anything else) always copy it somewhere as a backup. It's also much easier if you use an editor that has the smarts to layout and highlight different file types, e.g. Notepad++ on Windows, Text Wrangler on OS/X and Vim on Linux.
Another thing I do is to include a backup of the configuration folder in my daily backup process, which also includes my calibre libraries.
BR