Directly replying to myself:
I just read this thread, especially this post, stating that the hAP uses the same chipset.
Might that be related / the same problem? of course, my internet connection does not support Gb/s speed, syncing at a lower speed…
Even though I had a stable connection with this setup:
HP switch ← 1Gb/s → Mikrotik ← 100Mb/s → Fritzbox as router (with DMZ for mirkotik) ← 50/10Mb/s → VDSL
and this connection was unstable (port flapping / flopping):
HP switch ← 1Gb/s → Mikrotik ← 1Gb/s, PPPOE sync at 50/10Mb/s → Draytek Vigor 130 ← 50/10Mb/s → VDSL
This does support the explanation given later in that thread, that it is a software related problem. The switch chipset only saw connections with 1Gb/s in the second setup, but the port was flapping/flopping.
/axc27