Ah, I found the problem, and it was (sort of) user error. I was using a different SMTP plugin to do a very specific thing, and with the old Welcome Email Editor, it worked just fine. When I installed 6.1.1, it wanted to be my one and only SMTP plugin, and having the other one there too was what actually led to that error. So, I have everything working now (thanks), but just as a data point, I never needed or wanted SMTP features from this plugin, and I would have preferred to keep those elements in separate plugins.