@Jean-Christophe_Vroy You might just need to “Disable the Incoming Mail scanning” in Avast as in this eM Client thread below which worked for another eM Client V8 user.
eM Client 8 Avast work-around - Mail - eM Client
WKeat
Jul '20
Hi,
Avast work-around
Protection > Core Shield > Settings (gear on the right) > look for Mail Shield > disable Scan inbound emails
eM Client 8 should work
Hope these helps.