You should not need to downgrade the extension, the code that UE 4 produces will send emails in the way that it is configured. It is possible that there is some error in the way the email code was written to the page or a configuration error. I would like to see the page with the UE on it along with a copy of the webassist folder from the site. I can test this out to see what is going on. Also if you can reply to some of the questions from the previous post that info may help as well.
As for the errors you are having if clearing out the cache file does not resolve the problem you will need to clear your configuration folder. Here are links to the steps for both of these:
clearing the cache file
clearing the configuration