Once Safari is open, please navigate to 'Safari' in the top tool bar and click on 'Clear History and Website Data' once complete, please close Safari completely and reopen and navigate to the control panel at https://cp.everycloudtech.com.
The Flash control panel is still available here - https://control.everycloudtech.com.
If you still can't log in, then please attempt to load in an additional browser and let us know the outcome at firstname.lastname@example.org as this would then be machine specific / browser machine specific as we can enter and display the Control Panel without an issue using Safari on Mac.
Please remember that Safari on Mac is a little unique in that Safari specifically requires that Website data is allowed within the browser's security settings. After you have completed the above first steps you may once logged in need to navigate to 'Safari' on the tool bar -> 'Preferences' -> 'Privacy' and click under Cookies and websites data: Always allow.