For those experiencing similar Outlook connection issues, we were recently told that this might be related to MDM profiles installed by your workplace:
My work calendars and reminder lists don’t appear in Agenda
If your Mac, iPhone or iPad is managed by your work through so-called mobile device management (MDM) profiles, it might prevent Agenda from accessing your work calendars (e.g. from Office365 or Exchange). To verify whether this is the case, go to Settings > General > Profiles to check if you have indeed an MDM profile installed and other it has the following specific restriction listed: “Opening documents from managed to unmanaged apps not allowed”. If you have an MDM profile with this restriction installed by default it will not allow sharing of your calendars with Agenda.
To resolve this, you will need to speak with your workplace’s administrator, who will be able to grant Agenda the necessary clearance it needs to access your managed calendars. Specifically, Agenda either needs to be added as a managed app in the MDM profile, which allows it to access iOS’s calendar data, or the “Opening documents from managed to unmanaged apps not allowed” restriction needs to be removed from the MDM profile.