My apologies for the late reply on this, we went through the following steps to resolve the issue:
- Go to the Safari menu in the upper left corner of your browser.
- Select Preferences. Select Privacy. Under Cookies and website data, select Details (or Manage Website Data).
- Search for Salesforce.
- Select Remove.
- Select Remove Now.
- After the Salesforce website data has been deleted, force-quit Safari and try logging in to Salesforce again.
