WebSocketException: Connection to was not upgraded to websocket

[pnkmem429]

E/flutter (15940): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: WebSocketException: Connection to 'http://abcd.com:0/ws/#' was not upgraded to websocket
E/flutter (15940): #0      _WebSocketImpl.connect (dart:_http/websocket_impl.dart:1011:41)
E/flutter (15940): #1      WebSocket.connect (dart:_http/websocket.dart:320:22)
E/flutter (15940): #2      new IOWebSocketChannel.connect (package:web_socket_channel/io.dart:81:28)
E/flutter (15940): #3      connect (package:web_socket_channel/src/_connect_io.dart:15:24)
E/flutter (15940): #4      new WebSocketChannel.connect (package:web_socket_channel/src/channel.dart:115:16)
E/flutter (15940): #5      new WebSocketProvider (package:m2_ua/data/provider/base/data_provider.dart:15:33)
E/flutter (15940): #6      new RecordPerformProvider (package:m2_ua/data/provider/record_perform_provider.dart)
E/flutter (15940): #7      RecordPerformRepository.insertSet (package:m2_ua/data/repository/record_perform_repository.dart:51:20)
E/flutter (15940): #8      HMIController.savePerformSet (package:m2_ua/feature/hmi/main/hmi_view_model.dart:621:49)
E/flutter (15940): #9      HMIController.savePerform (package:m2_ua/feature/hmi/main/hmi_view_model.dart:535:11)
E/flutter (15940): <asynchronous suspension>
E/flutter (15940): 
I/flutter (15940):     ------------------------[WebSocket: onError]----------------------------
I/flutter (15940):     WebSocketChannelException: WebSocketChannelException: WebSocketException: Connection to 'http://abcd.com:0/ws/#' was not upgraded to websocket
I/flutter (15940):     

I was using well until last Friday, but today, suddenly these error logs come out everywhere.