K9 on Android 13 - missing notification for 1 of 4 mail accounts

Hi guys,
K9 is working a long time on my Android Phones so that I have a lot experience with it. Presently I use it on my Samsung Galaxy S20 FE under Android 13.
I’ve set up 4 different mail accounts where K9 retrieves mails via IMAP. Four K9 widget envelope icons placed on my home screen, each for one mail account. Further I do not use a common inbox for all of the three accounts but handle all separately.
And as long as I used only 3 mail accounts, all worked properly. But since a added another new mail account (now 4 total) I’m getting Android notifications only for 3 of the accounts. And I don’t miss the notification for the mail account which I’ve added recently but for another account which worked until now.
What I still get for all 4 accounts is the bubble containing the number of new mails next to the envelope icon on my home screen. But any sounds, notification on lock screen or status bar notifications are missing for one account. I’ve checked all the Android permissions for K9 and found them comformable to the other 3 accounts.
Has anybody an idea?