Samsung Keyboard problem (disappearing while typing)

Hi guys,

I just switched from a Galaxy S3 to a Note 5 N9208 and I'm very happy with the Note so far, it's a really great device.

BUT... I'm having a really strange problem. It doesn't matter whether I'm typing in whatsapp or in the Google Chrome browser, the Samsung Keyboard seems to "crash" (or simply disappear) randomly. Yesterday it happened about 5 times while writing a friend on whatsapp. The Keyboard disappears and sets me back to the Chat menu, it's the same as if I would press the back button twice.

I'm using the german Keyboard (qwertz), but it also happened on the UK Keyboard before... also I have updated the Keyboards in the Settings menu.

I'd be very thankful for any assistance :)