@cedricundert Please share the output of M122 as well. If the RasPi isn't always available via ping, I'd tend to say there is a problem with your router or PC. I am not aware of any network problems with version 3.5.1.

@e4d another possibility is that the large number of http clients is causing RRF to run out of output buffers. On the 6HC we have enough spare RAM to increase the number of buffers.