I have only found (so far) one problem which may or may not be related to the above error I received when I upgraded last night; - My TLS (STARTTLS) doesn't work....
Fortunately, thanks to some kind posters, I found this solution which WORKED!
TLS not being offered on SMTP after upgrade from 5.0.18 to 6.0.0
Other than that, things are good so far...
PS It is possible that I received the error because my dns server (just found it) was not reverse-looking up with the correct hostname...