That didn't work: a BT keyboard will not connect

[abmvk]

I am running a Raspberry Pi 4B with Pi OS (Debian). I use a mouse over USB and a keyboard over Bluetooth. Works fine. But I wanted to use a smaller keyboard with a touchpad. It pairs, but then it doesn't connect. The error message I get is meaningless for me:

Connection failed - GDBus.Error.org.bluez.Error.Failes: br-connection-create-socket

Could someone put me in the right direction?