An Error occurred during online meeting - Some sharing features are unavailable due to server connectivity issues ...

External users can join online meetings and can share desktop without any issue. But if they try to initiate whiteboard, presentation, or polls, Lync/SfB client shows:

An error occurred during an online meeting. Some sharing features are unavailable due to server connectivity issues.

The OCLogger on a Front End server shows:

Start-Line
: SIP/2.0 488 Not acceptable here
3041;reason="Participant is not connected to conference" ;source="FE01.domain.local"

Lync logs showing:

54001; reason="Connection to the Web Conferencing Server could be established, but the server terminated immediately"; ClientType=Lync; Build=4.0.7577.4456;
Branch=lcs_se_w14_main; ContentMCU="sip:user@domain.com; gruu;
opaque=app:conf:data-conf:id:G6C6JNHM";
ConferenceUri="sip:user@domain.com; gruu;
opaque=app:conf:focus:id:G6C6JNHM"; ServerFQDN="lyncwebconfedge.domain.com";
ServerPort="443"; ConnectionMethod="DataProxy";
ServerIPAddress="xxx.xxx.xxx.xxx"; ClientIPAddress="192.168.2.63";
ConnectionType="Wired"; SocketCode="10054 (WSAECONNRESET)"

The firewall Team confirmed, all the required ports are open - however, on testing it is identified that port 8057 between Edge Servers and Front End Servers is blocked. The port 8057 is used for Web conferencing traffic from Front End Server or each Front End Server if in a pool, to Edge Server internal interface. Once the port is configured correctly, the whiteboard, presentation, and polls started working without any issue.