So, today iam back at my parents home and they use a TP-Link WA730RE Range Extender and my iPad works perfect but my MacBookAir wont. So after many many many days and searching and turning the RE off at the end… i thought there is no way to solve this Problem.
But then i remember that this Rangeextender has a Webinterface too which is sadly only available with a network-cable.
So ive set my IP hard to 192.168.1.100 / 255.255.255.0 and conntect to 192.168.1.254 admin/admin.
There, ive turned the protocoll and the encryption hard to WPA2 and thats it (instead of let the repeater choose between different/all possbilities).
Now everything works perfect!