I notice in one of your old threads, where you had also this problem in V8.2 at that time after the automatic account setup completed and getting timeout errors, you stated you had no optional firewall installed but had Avast and PopPeeper installed.
Do you still run Avast and PopPeeper ?
If you do still run those, try “completely disabling those to test incase of interference”. Avast in particular has a “Mail Shield” enabled that can block email receiving even if you add eM Client as an allowed program.