ok i have a solution:
enter the sd-card in your pc and open the boot partition. add the line “init=/bin/bash” to the file cmdline.txt in the root directory. now start up the pi again with the sd-card. now, you should be able to get a ssh connection and fix the problem by apt-get removing all the packages that could have caused your problems.
@osm-staff: thx for not helping at all with this problem. i really am a bit disappointed i have to admit. as a not very expierienced linux-user i guess this is quite a basic problem if even i was able to figure that out (after a week of hoping for answers).