Hello Stefan. Welcome to Android Forums.
I'm wondering why you're being prompted that the free version of ROM Manager is not available, as I see it just now as I check for it in Android Market.
I'll suggest that perhaps a cache and data clearing of the Market app (sometimes shown as "vendor.apk" in some versions of the Andriod OS), via application manager, then reboot and try Market again for the free ROM Manager. You'll have to "sign" the user agreement again, if you do clear data as well as cache.