From time to time my K9 will crash on start. Sometimes restarting the tablet would allow it to start.
Other times I would killl the process then restart. If that didn’t work I would try a clear cache before restart.
Last resort was to reinstall … I download the app to by pass play store, so I have it on hand for the reinstall.
As my instance would run with the same set of emails and folders, I dont think it was a particular message that caused the crash on start
But knowing how to find the local storage to remove some sounds like a good thing to know.
My issue was with 5.704 And some times apparent in. 5.716 & 5.717 on Android 8
This is the only problem I have had with K9, now I have my settings exported to make resetting easier!
Asking where the local storage is, and if the odd message can be deleted or if there is an index file that can be deleted and recreated would be good to know.
Best of luck!