Hi @Per-Oyvind_Odegard ----- Thank you for the update and sharing the observations made during your testing. The continued insight is very appreciated!
Continuing forward I noticed from your network topology description that you are making use of HP Procurve 1800-8G switch. If I am not mistaken this a managed switch and we have seen a few issues crop up with these devices in the past ( Example: #1, #2, #3). I would recommend enabling flow control on the switch and confirming if this triggers a change in behavior with Chord Poly as we have seen it help other in the past.
-Eric