Investigate cause of multiple error messages

Error log messages

• DefaultHandshakeHandler: Handshake failed due to invalid Upgrade header: null
  Handled by !26 (merged).
• StompBrokerRelayMessageHandler: Received ERROR {message=[Subscription not found], content-type=[text/plain], version=[1.0,1.1,1.2], content-length=[53]} session=5ade7953-... text/plain payload=There is no current subscription with tag 'T_sub-11'.
• StompBrokerRelayMessageHandler: Received ERROR {message=[No subscription found], content-type=[text/plain], version=[1.0,1.1,1.2], content-length=[92]} session=27d3826a-... text/plain payload=UNSUBSCRIBE must refer to an existing subscription.

Possible causes

• Client side
  • Additional STOMP subscriptions per client (arsnova-webclient!946 (merged))
  • Updated StompJS/RxJS library
  • Browser changes: https://github.com/stomp-js/stompjs/issues/335, https://developer.chrome.com/blog/timer-throttling-in-chrome-88/#intensive-throttling (Chrome)
  • Outdated browser which has WebSocket support but might only support an older spec which isn't feasible for StompJS.
• Server side
  • Updated Spring dependencies (unlikely, only patch release updates)