From the looks of it all I can think of is something must have happened to your host memory ( Speicherzugriffsfehler (Speicherabzug geschrieben) ) , or there had to be some kind of update/upgrade between the time the product worked and when it started to fail. As for removing the two new guests since you can not use the main manager you would need to use VBoxManage to remove them ( see the users guide for the syntax ) or as a last result you could modify the virtualbox xml file to remove them ( be sure you make a backup ) You could also do as mpack suggested and delete the xml as virtualbox will create it for you ( empty ) and you can use the add feature of the main manager to add the guests you want back in.
Note 1: I did research your segfaults you posted and so far have not found anything related to virtualbox. If this is in fact an issue you are the first.
Note 2: If you need to get back to work soon you can always try to start your guest/s from command line. It should work since you are not using the main manager which is the only thing that I know of that actually uses QT.