I have several devices based on ESP8266 which have been running perfectly until I installed a set of Cisco 140AC access points. Being Cisco I assumed they would be trouble free but they have upset the ESP8266.

The devices all have a small WEB site on them to allow me to configure them for WiFi credentials and to manage the things they control. They also provide data in the form of JSON via a number of api end points. I find the when they are connected to the Cisco 140AC they will happily provide the JSON data but I cannot access the configuration or management pages. I found that it seems to be page size related. I am using the response.sendFile() call to transmit an HTML page to the client and if I keep the size of the page down below approximately 1500 bytes all is well. As soon as I try to send more than that the ESP8266 fails to send anything and the connection times out on the browser requesting it. This does not happen with my older Engenius access points or when I am directly connected to the ESP8266 running in access point mode.

Everything else I have seems to work fine with the Cisco access points, just the ESP8266 is failing.

I tried adding a bunch of debug print output to various library functions and it would seem that after the stream is sent to the client there are a bunch of missing entries when the size of the packet is too big.

This is a sample from a transmission that works...
`
[:135] TCP 3fff2780 timeout updating: 70 -> 2
[:97] Opening connection. Total connections: 2
[:107] TcpServer onClient: 192.168.10.127

[:528] internalOnSent
[:154] onSent
[:155] TCP 3fff2488 sent: 1564
[:158] TCP 3fff2488 4 64
[:205] onReadyToSendData
[:207] State : eHCS_Ready
[:168] TCPClient::readyToSendData(2)
[:212] onReadyToSendData
[:214] TCP 3fff2488 onReadyToSendData: 2
[:174] No delegate sourceEvent:ready
[:196] Stream is null
[:532] TCP 3fff2488 <sent
[:538] staticOnPoll
[:551] internalOnPoll
[:167] TCP 3fff2488 TcpConnection::onPoll() sleep = 1 : timeOut = 2
[:174] TCP 3fff2488 1 - 2
[:205] onReadyToSendData
[:207] State : eHCS_Ready
[:168] TCPClient::readyToSendData(3)
[:212] onReadyToSendData
[:174] No delegate sourceEvent:ready
`

Tihs is the same section from where the transmission fails...

`
:135] TCP 3fff2928 timeout updating: 70 -> 2
[:97] Opening connection. Total connections: 2
[:107] TcpServer onClient: 192.168.10.127

[:538] staticOnPoll
[:551] internalOnPoll
[:167] TCP 3fff2508 TcpConnection::onPoll() sleep = 1 : timeOut = 2
[:174] TCP 3fff2508 1 - 2
[:205] onReadyToSendData
[:207] State : eHCS_Ready
[:168] TCPClient::readyToSendData(3)
[:212] onReadyToSendData
[:174] No delegate sourceEvent:ready
`

The file names are missing from the debug output due to another bug which I logged the other day but the line numbers are useful.
The ESP8266 remains responsive and does not seem at all phased by the failure to transmit the packet. If I try to send a string immediately after the file the string is sent to the client correctly but there is no file.