FontExplorer X will run fine on a machine with Leopard installed as an "archive and install," but will not run properly if installed on a clean Leopard installation.
I get an error message stating that permissions are probably set wrong, possibly in parental controls. Parental controls are turned off, and I have the problem whether logged in as a standard or administrative user, whether FontExplorer is installed in the Applications folder or my home folder.
I'm assuming the problem is that FontExplorer installs a component in a directory even administrators don't have write access to, but I can't figure out what that component is. A Spotlight search on "Linotype" or "FontExplorer" turned up empty on a machine with a working install of FontExplorer
Also, I should clarify: I have two machines running FontExplorer which had been installed before I did an archive and install of Leopard, and FontExplorer works fine on those machines. The problematic machine had FontExplorer installed after a clean install of Leopard, and it won't run on that machine.
I can certainly understand your frustration with this problem. I have done some checking on linotype's website and also versiontracker and neither site states that the program will run or is certified for Leopard. However, your mileage may vary, as you have already found out. My suggestion for the clean install leopard machine is to wait until linotype has released a version certified for leopard and then try again. I know it may not make sense as to why the computers who archived and installed will work and why the clean install won't, but there are literally thousands of reasons why when you start looking at the architecture of the file system and what gets moved where during an operating system upgrade.
Good luck! If you are not satisfied with this answer, let me know and I'll open the ticket back up for someone else to take a look.