There was a spate of similar problems reported here a few weeks ago. It turned out that all the people reporting the problem had accounts with the same company, I think it was called GoDaddy. Further research showed that many of their customers were reporting this even though they were using other email clients than Thunderbird. (Eventually the company fixed the problem, I believe: it was something to do with their servers.) So don't assume that it's due to the Thunderbird update, especially since (as you say) it seems to be working well otherwise. I'd suggest you look on your email provider's website, see if they have a help forum, if other users are having the same difficulty.

Thanks