Rarely, one of the files gets corrupted.
Try deleting files under Troubleshooting at the bottom of the main page.
If Calendar will not open, try deleting the Activities file.
If Open TIme will not open, try deleting the Open Time and/or the Activities file.
Sorry for the trouble.
I have not been able to ascertain whether it is a program error, a network error, or a certain usage sequence that causes this.