I know that a project normally cannot be deleted, because there are Issues assigned to it. But there is still a possibility to erase a project over the webinterface, which brings the system into an inconsistent state:
In a theoretical case where we move all issues to a different project to erase the old project, the assignment PERSONKEY/PROJKEY in the table TACL is not erased.
Track+ crashes when loading the user specific acess control lists at logon.
Please consider this in the next release.
I have copied your post to the bug list. There will probably be a bug fix release soon.