I installed the latest K-9 on the Tab S9 FE a few days ago (before 6.803), exported the configuration from a working Pixel7, imported the configuration, and set passwords. Everything worked fine except for Push notification. I followed the instructions at https://docs.k9mail.app/en/6.400/settings/push/ and also disabled battery optimization for K-9. The issue is similar to https://forum.k9mail.app/t/push-not-working-on-android-14-k-9-mail-6-800-6-801/8181 except I was able to enable “Alarms and reminders” (no improvement). I also noticed that K-9 “General Settings” only showed Account Name and something related to color.
In checking forum posts, I found a suggestion to set the “Folder poll frequency” to something other than Never. This resolved my Push issue and “General settings” now showed the full list of options. After reverting “Folder poll frequency” back to Never, Push worked for a while but then stopped again. I set “Folder poll frequency” to 15 minutes, successfully tested Push, switched back to Never, and have not had any issues since.