Incoming mails from GMX and WEB.DE are empty

[stphn]

Did you try to restart your phone after this happened?

Message text not displayed

K-9 Mail doesn’t check the bluetooth state to decide whether to display a message. It feels very unlikely that turning off bluetooth has any effect on this. It’s more likely that somehow the WebView component on your device (that K-9 Mail uses to display the message body) stops working and a reboot will fix the issue, at least for a while. Make sure you’re running the latest Android version available for your device and contact your device vendor if this problem persists.

Ignore the part about bluetooth but restart seems to solve what you see.