Websocket handshake error 3

[phpony]

So, what's the current state of this issue?

"" "[ Service ] [2018-09-08T11:27:34] debug: connecting websocket: pubsub1.cloud.symless.com:443" 
"" "[ Service ] [2018-09-08T11:27:35] error: websocket handshake error 3: WebSocket upgrade handshake failed" 
"" "[ Service ] [2018-09-08T11:27:35] error: websocket handshake response 400: Bad Request" 
"" "[ Service ] [2018-09-08T11:27:35] debug: clearing last profile snapshot" 
"[ Config  ] " "[2018-09-08T11:27:35] debug: service cloud connection error" 
"" "[ Service ] [2018-09-08T11:27:35] debug: retrying websocket connection in 6s" 
"" "[ Service ] [2018-09-08T11:27:41] debug: retrying websocket connection now" 
"" "[ Service ] [2018-09-08T11:27:41] debug: connecting websocket: pubsub1.cloud.symless.com:443" 
"" "[ Service ] [2018-09-08T11:27:42] error: websocket handshake error 3: WebSocket upgrade handshake failed" 
"" "[ Service ] [2018-09-08T11:27:42] error: websocket handshake response 400: Bad Request" 
"" "[ Service ] [2018-09-08T11:27:42] debug: clearing last profile snapshot" 
"[ Config  ] " "[2018-09-08T11:27:42] debug: service cloud connection error" 
"" "[ Service ] [2018-09-08T11:27:42] debug: retrying websocket connection in 24s" 
"" "[ Service ] [2018-09-08T11:28:06] debug: retrying websocket connection now" 
"" "[ Service ] [2018-09-08T11:28:06] debug: connecting websocket: pubsub1.cloud.symless.com:443" 
"" "[ Service ] [2018-09-08T11:28:06] error: websocket handshake error 3: WebSocket upgrade handshake failed" 
"" "[ Service ] [2018-09-08T11:28:06] error: websocket handshake response 400: Bad Request" 
"" "[ Service ] [2018-09-08T11:28:06] debug: clearing last profile snapshot" 
"[ Config  ] " "[2018-09-08T11:28:06] debug: service cloud connection error" 
"" "[ Service ] [2018-09-08T11:28:06] debug: retrying websocket connection in 14s" 
"" "[ Service ] [2018-09-08T11:28:20] debug: retrying websocket connection now" 
"" "[ Service ] [2018-09-08T11:28:20] debug: connecting websocket: pubsub1.cloud.symless.com:443" 
"" "[ Service ] [2018-09-08T11:28:21] error: websocket handshake error 3: WebSocket upgrade handshake failed" 
"" "[ Service ] [2018-09-08T11:28:21] error: websocket handshake response 400: Bad Request" 
"" "[ Service ] [2018-09-08T11:28:21] debug: clearing last profile snapshot" 
"[ Config  ] " "[2018-09-08T11:28:21] debug: service cloud connection error" 
"" "[ Service ] [2018-09-08T11:28:21] debug: retrying websocket connection in 21s" 

I've tried the beta in 2017, it wasn't working. Tried just today (fully upgraded Debian Buster, freshly installed synergy-core 2.0.12, protocol version 1.6) - still not working.

I have a PC running Windows 10 in the same network, and it's working just fine. So, it's obvious a Linux-specific bug not being addressed for a year already?

[phpony]

Read out all those topics, can't find any working solution: