After quite some struggeling getting this to work (including problem with internet over usb following this guide Sharing internet using Network-Over-USB in BeagleBone Black ), I got also a working solution for the Wifi.
I followed the guide at TP-Link TL-WN722N USB wireless adapter Beagle Bone Black with one minor typo (it is /etc/network/interfaces) and added three lines to my /etc/rc.local (as root, in front of exit 0 line).
connmanctl disable wifi
connmanctl enable wifi
The reason for this is because my adapter always started in master mode and not in managed mode.
Remark: This is most probably not the proper way of doing things, but it worked for me.