Firebird 1.5 on Ubuntu Dapper - How to fix "operating system directive open failed" error

After installing Firebird 1.5 on Ubuntu Dapper I ran into a strang problem. I first installed firebird2-classic-server and everything worked. Then I installed firebird2-dev and suddenly I couldn't connect anymore to my database, not even locally as root. If the fixes in the article above doesn't help you, make sure that the /usr/lib/firebird2/run symlink points to /var/run/firebird2, and that the directory actually exists! A simple mkdir /var/run/firebird2 as root did it for me. The big question now is: Why did this directory disappear in the first place?