I believe the problem is the inbuilt settings you have in Aquamail. With the recent change with the updates app, Aquamail has moved to the Microsoft Graph API which is newer and will be supported by Microsoft long into the future (hopefully!) Microsoft will eventually retire Exchange (EWS) API.
I would recommend that if you choose to keep the current app version installed, to remove your current account and re-add it. You could also do a backup of the app settings only, then clear App data and cache, and restore the backup. Add your account again after that as Office 365. It will likely now pickup that it's Graph API. I had this error happen to me after I authenticated after the app updated to the newest version.
Note that currently Aquamail has bugs with Graph API that they are working on. Personally, while I can send and receive emails, any movement of emails (archiving, deleting, moving, etc...) only result in the server marking them as read. On my phone they appear to move but on the actual server they stay in the inbox. There are others that have remarked that there are issues currently with calendar events and syncing as well.
In the meantime what I've done is uninstall the app and I downloaded the prior version from APKmirror. In play store turning off automatic updates. This allows me to keep using the EWS version of the API while I wait for news of them fixing the Graph API bugs.
Hope this all helps!
