Hello ,
I have added debug log of kamailio for better understanding of this issue.  i am seeing logs like this for every ACK sent from SIPML5.

Error:
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/receive.c:245]: receive_msg(): --- received sip message - request - call-id: [954f1f9e-f0c1-dfc5-fbb2-e3fcbdd40208] - cseq: [56855 ACK]
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/receive.c:295]: receive_msg(): preparing to run routing scripts...
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: sl [sl_funcs.c:416]: sl_filter_ACK(): SL local ACK found -> dropping it!
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/usr_avp.c:636]: destroy_avp_list(): destroying list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/xavp.c:495]: xavp_destroy_list(): destroying xavp list (nil)
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: <core> [core/receive.c:457]: receive_msg(): cleaning up
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[1668iúaÞ*ÍK¹h¯9i¥3‡{­$Ô8óqÚ*âm›¡-rad.c:1243]: ws_process_msg(): WebSocket Message: [[>>>#012 þ#0ið%Çs®7v©%Òx² ‡y¨|“#¨#006Ýx‹,¨_¥yîpÝ(§5Ü*îpÜ(à# ‘7.¡3sÇ*lŠv©$‡nï#016¤[ñoÙ:³(™WŒtÄlñoÛ*ñwÇ*óoÙ.ÍK¦h§ ‡sº s¯/Ó:i©1Ó.ðuÙZörÇ+ôsÇ(õqÇ+õsË6²$j¯/ýc‹/òq-ñ"Š*ðtÙ)óuÐ)òr+øpÙ|¥vÙ*ùvË6¡-u²( r­|¤^õLãO³$›7 &Œt´{ÉS                                                      .œx¡/uà#025Œv¥"wÍKä#020¶|Ù#027Ê.Ôw¯;€v¬ Ç4î#025¡S“#036 IŸ#022­J #023½[íwÝ4ðaÛ.ùsÝ"õuÞ"÷tÚ/òyÙ*ðaÙ:‰#017ÉSx¡/uà#025Œv¥"wàlÉ|©3Œ|¯9ä#020´|Ù:ðLã{ý2Œt¤3Œy¶Lã{ý'€t§$›j²(‡nú2 {ísÜ,àwÛ ñpÓ"÷{¨\ú#002ª 
sÓ#÷{¬,úyØ ò#007Ó/÷{Ý^úsÙ ó<<<]]
Jan  9 07:55:37 client1 /usr/local/sbin/kamailio[16685]: DEBUG: websocket [ws_conn.c:438]: wsconn_get(): wsconn_get for id [1]

PFA.

On Wed, Jan 9, 2019 at 7:08 PM Ravindrakumar Bhatt <ravindra.bhatt@ecosmob.com> wrote:
Hello,
I am using latest stable kamailio version 5.2 . I am getting following error while calling from webrtc client to sip client.
Error:
Jan  9 07:25:05 client1 /usr/local/sbin/kamailio[15318]: INFO: <script>: ACK rsponse: WebRTC --> WebRTC
Jan  9 07:25:05 client1 /usr/local/sbin/kamailio[15318]: WARNING: <core> [core/msg_translator.c:2812]: via_builder(): TCP/TLS connection (id: 0) for WebSocket could not be found
Jan  9 07:25:05 client1 /usr/local/sbin/kamailio[15318]: ERROR: <core> [core/msg_translator.c:2003]: build_req_buf_from_sip_req(): could not create Via header
Jan  9 07:25:05 client1 /usr/local/sbin/kamailio[15318]: ERROR: <core> [core/forward.c:549]: forward_request(): building failed
Jan  9 07:25:05 client1 /usr/local/sbin/kamailio[15318]: ERROR: sl [sl_funcs.c:372]: sl_reply_error(): stateless error reply used: I'm terribly sorry, server error occurred (1/SL)

Here it is saying that websocket connection is not found but when i check using netstat connection is in established state.

netstat -anp | grep kamailio
result :
tcp        0      0 XX.XX.XX.XX:3080     0.0.0.0:*               LISTEN      15324/kamailio      
tcp        0      0 XX.XX.XX.XX:3080     0 XX.XX.XX.XX:53632   ESTABLISHED 15298/kamailio  
53632 is same port as port mentioned in received field of location table.
So what can be the issue ? please provide your feedback on same.

--
Thanks and Regards,
Ravindrakumar Bhatt
Jr. Software Developer 
Ecosmob Technologies Ltd
Ahmedabad
Mo:+918460692402


--
Thanks and Regards,
Ravindrakumar Bhatt
Jr. Software Developer 
Ecosmob Technologies Ltd
Ahmedabad
Mo:+918460692402