Hello Duncan,
My name is Jackson and thank you for your question.
I sincerely apologize for the issue you experience with your outlook.
First we can try using outlook on the web to see if we experience similar issue on the web https://outlook.office.com/
If we can use outlook on the web kindly try to create a new profile, and add your account to the new profile and check if the same error still occurs;
Depending on your deviced see details in the link;
https://support.microsoft.com/office/f544c1ba-3352-4b3b-be0b-8d42a540459d
For Mac; https://support.microsoft.com/office/fed2a955-74df-4a24-bef6-78a426958c4c
Warm regards.
Jackson E.