Thanks a lot for your help, but unfortunately I'm still getting the same error.
I know that Microsoft was disabling basic authentication for all protocols (except for SMTP) starting October 1st, and apparently it struck me now.
Here's the Microsoft article about it:
https : // learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/deprecation-of-basic-authentication-exchange-online
After taking the recommended actions to re-enable basic auth for EWS (see attachment), all was working fine again. But this is only going to be a temporary solution, because Microsoft will finally disable all basic auth by the end of this year and you won't be able to opt out anymore.
I'm sure this will affect basically every O365/AquaMail user within the next few days, so let your Exchange admins know!
AquaMail Team: Please provide a solution otherwise AM will simply become unusable for O365 accounts.