I recommend that the very first thing you do be to initialize preferences.
Here are the steps:
Quit the sequencer.
Go to “~/Library/Preferences/Cubase 5” (where ~ is your username on your mac), this folder contains all preference files of your sequencer.
Rename it (for example to “Cubase 5 hidden”) in order to hide it from Cubase, or move it somewhere else that’s handy. You even can simply trash it if your user settings are not that hard to rebuild.
Next time you start Cubase it will create a new folder with fresh preferences and default settings - which hopefully fixes the problem.
Another approach: Since in many cases the file Defaults.xml will be the trouble maker you could also try if renaming only this particular file already solves the problem.
Note: If you find folders of previous Steinberg sequencer installations make sure to hide them too by renaming or removing them. Otherwise the sequencer would try to comply with them when creating fresh preferences. And exactly this could be the source of any error.
Initialising the sequencer’s preferences does not necessarily mean that you lose all settings. For example if you have defined your own set of key commands that you want to keep you can replace the corresponding file (“Key Commands.xml”) in the freshly created folder by a copy of the same file taken from the folder you’ve disabled by renaming.