I am transitioning from a Samsung Tab A (7", Android 5.0.0) which works perfectly with K9, to a Lenovo TB-8304F1 running Android 7. K9 version is 6.301 in both cases.
On the Lenovo, I get a consistent crash every time I try to read two specific emails. They both have “click for more” messages, but they crash K9 before clicking those; merely selecting the email causes a brief flash of the message, then K9 dies (images not displayed by default on all messages).
The two messages are both daily mailing-lists, one from the NYTimes, and the other from Digital Music News (easy to subscribe to the latter at www.digitalmusicnews.com). I can forward either message to some address if necessary.
I don’t know where to look for debug log messages, but if told where/how I can give it a peek. K9 performs perfectly on my old Samsung Tab A with Android 5, and also on my Motorola Z2Play with Android 8.
There’s a few things missing from the lines here, but it looks like a native crash. There is not a lot of native code in K-9, as far as I know. Try updating the AndroidSystemWebView app. If you have disabled the app, re-enable it.