Text in header and timescale are invisible and when attempting to enter an activity, the row height becomes very large.

This is behavior that a few users who have migrated user accounts between Macs have experienced.

In order to resolve the issue, do the following:
1. Go to Applications > Font Book
2. Under "Collection", select User
3. Right-click (or Ctrl + click) on one of the fonts on the list
4. Select Reveal in Finder
5. In the opened Finder window, you should be in a Fonts folder
6. Create a new folder on your desktop
7. Go back to the Finder window displaying Fonts and select all of the fonts listed
8. Transfer all of them to the new folder on your desktop (Fonts folder should be empty)
9. Transfer all of the fonts back to the Fonts folder
10. Relaunch FastTrack Schedule

Basically, you need to reload your User fonts.