Thanks so much for the help to everybody!
Latest ping result with Zyxel P2812 AC (5 ghz band just like on Aruba):
Ping statistics for 192.168.10.1:
Packets: Sent = 300, Received = 300, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 1ms, Maximum = 19ms, Average = 1ms
Both AP use the same sec. settings (WPA2, no mixed). U-APSD was disabled when testing with both AP.
Looks like a different AP and regfix (ScanWhenAssociated) solved the problem.