Hello Cyberzork,
Actually I found it was due to emClient trying to lookup my domain autodiscover settings from o365 (althought I never used the domain on o365 infrastructure…) and this was preventing autodiscover to work correctly with an EWS enabled account.
I added
127.0.0.1 autodiscover.outlook.com outlook.office365.com
To my hosts file and now autodiscover works again like a charm.
It would still be interresting to know what emClient is asking to outlook.office365.com and what is answered, and why it prevents autodiscover process to succeed.