If you are experiencing issues adding your account to Calendars, please check the following:
If the steps above didn't help, try connecting your account through local calendars:
Note: If you already have some of the accounts added to Calendars by Readdle and don’t want them to be added twice to the app, disable them in the Local Calendars section of Settings (⌘,). Disabled accounts will still be available for other apps and systems.
Note: If you add Exchange or Outlook Calendar through local calendars, color-coding will be random for these accounts.
Tip: To learn how to add multiple accounts to Calendars on your iPhone or iPad, check out this article.