@ohne It looks similiar to this case: https://community.tp-link.com/en/business/forum/topic/508722

So tplink has found the issue and confirm the statistics of Error/Dropped packets are incorrect, it won't affect the actual Wi-Fi performance

Thank you for the hint and your answer. Since there are several real wifi problems, even with an Echo device which is like 1m away from the AP I don't think that this is just a problem in the interfaces. The amount of errors do reflect the bad wifi quality. Like I said, Spotify on the Echo Dot drops the connection every 45-60 seconds and video calls get interrupted because there is so much packet.

Just checked again: No it's even 36% packert error on 2.4Ghz...

@ohne It's getting worse, nearly 40% - device is unusable like this :(

Edit: After a factory reset of the EAP660HD and nothing changed in the configuration tx error rate jumps directly to 30-40%. 

Tx Packets:

23383

Tx Dropped Packets:

8423

Unfortunaley I was wrong with my suspicion that the problem is vlan related. just added a simple router to the network without any vlans and just the eap 660 shows again round about 40% drop/error. I'm starting to believe that the unit is broken..

Alright I think I found the problem. It seems to be VLAN related. When I remove the VLAN tag from the SSID TX error/drops normalize to a, well let's say 'okay' level (5%). When I add the VLAN tag again it kind of explodes.

Perhaps someone has a clue why this is happening?

Setup is like this:

Netgear ProSafe

Port 1, native VLAN 1, Tagged : ESXI Host with opnSense (VLANs are managed by ESXI and added to opnSense as NICs)

Port 2, native VLAN 1, tagged : EAP 660 HD

Settings EAP: 

LAN VLAN 1

SSID 2.4Ghz: VLAN 20

SSID 5Ghz: VLAN 10

To exclude any other things, I added a simple wan gateway/router and removed all other devices and VMs. I also removed the AP from the Controller and installed it without a controller. I also changed the switch, cables and power adapter. Result: 40% packet loss and devices are still loosing connection (music and facetime drops on several devices). So perhaps it's defectiv or well I don't know what else... Will try to get a replacement..

Packet Drop/Error running the AP without Controller:

  @ohne 

Any news concerning the EAP660HD, did you get a new one?

  @Spryde 

Unfortunately it is not possible to get in contact to someone at TP-Link. Support tickets aren't replied and telephone support doesn't pick up after 20 minutes.

@ohne 

Ok - this is of course not good news.

Hat only short contact with their support - but got answers the next business day. Confusing that they don't give you any answer...

@Fae Could you give us a feedback concerning the EAP660HD drops/errors on RX/TX 

Even if it is only a bug within Firmware (you said it somewhere else here), it brought my switch to go down to 100MBit/s as every peak in TX/RX dropping gives internal traffic spikes and then a jump from 1000 to 100MBit/s.

So it's a quite annoying bug. 

Any outlook for new firmware?

  @Spryde 

Have you tried this firmware? this is beta but I do not have anything like that you mention on either EAP660 or EAP620

https://community.tp-link.com/en/business/forum/topic/531760

This is 5Ghz but is simulare in 2.4Ghz