Sorry to hear your upgrade didn't go as smoothly as it should. This certainly isn't expected behaviour and we have received no other reports of this issue. Would you mind gathering a set of debug logs and sending to me via direct message? I'd be more than happy to take a look for any pointers as to why the e-mail connector stopped working.
If you have any additional feedback on 6.6, especially with regards to the UI updates, I'd love to chat!