Be sure that you don't have marked a range in the timeline's work area (gray bar over the timeline). If you press N, the end of the work area is changed to the CTI position so probably you have modified that.
Not sure but I'm afraid that there is no official support for AE 17 under macOS Big sur.
You can try to delete the preferences file. Hold COMMAND+OPTION+SHIFT on your keyboard as soon as you open After Effects. You will be prompted to delete the preferences file and try to preview a composition.