Microsoft confirms this misbehaviour in their Knowledge Base article number Q322237. They recommend that you "Move the Users folder back to the hard disk partition that contains Mac OS X -or- reformat your hard disk to have a single partition!"
However, after some searching the net, I found this workaround by Brad Hutchings that resolves the issue without reorganising your whole system. I tried this, it works flawlessly so far. Here is Brad's hint as posted in microsoft.public.mac.office.excel:
At least in my setup, Excel seems to have problems with my home directory being on a different disk partition than the root OSX partition. I found this out by creating a temporary user with a home directory on the OSX partition and found that this user could run Excel with no problems.[Editor's note: I have not tested this myself.]
So, here is what I did. I took the Carbon Registration Database file that Is created the first time you run any Office X program and deleted it from my home directory (it is in ~/Library -> Preferences -> Microsoft). I then copied the Carbon Registration Database file from the temporary user directory. I have been running this way for several days with no problems. Make sure you are not running any Office programs when you do this. YMMV, no doubt.

