Today I tried to create an item in calendars using Nintex on Office 365. I configured a service account and an user account to impersonate. But my 
workflow kept giving me:
The request cannot be processed due to internal error.
I found an article by Danny Ong. However Danny is using the admin@tenant.onmicrosoft.com account. Where I’m using my service service@tenant.onmicrosoft.com and I’m impersonating pieter.veenstra@mydomain.co.uk.
Once I changed my service account to service@mydomain.co.uk the problem went away.
My service@mydomain.co.uk has a calendar within our domain where service@tenant.onmicrosoft.com doesn’t have a calendar. This seems to be needed.