Broadcast sign-in fails

Hi there,

The case: User has been moved from on-premises Skype to Skype online: all good, client is working etc... That person go to Broadcast service and setup a meeting. Issues start when person tries to sign-in to the meeting as event member. Skype client says: "An error occurred during the Skype meeting".

On the tracing logs I could see the following:

04/13/2018|15:42:01.330 1CEC:1CF0 INFO  :: Data Received -10.11.12.13:8080 (To Local Address: 10.21.22.23:56831) 1018 bytes:
SIP/2.0 504 Server time-out
ms-user-logon-data: RemoteUser
Authentication-Info: TLS-DSK qop="auth", opaque="6AC09FAC", srand="7F163385", snum="3418", rspauth="16....8f1c", targetname="DB40E08FES10.infra.lync.com", realm="SIP Communications Service", version=4
Via: SIP/2.0/TLS 10.21.22.23:56831;received=10.31.32.33;ms-received-port=60324;ms-received-cid=11E52A00
Content-Length: 0
From: "FirstName o365"<sip:firstname.o365user@company.com>;tag=bb236a6679;epid=59887b3640
To: <sip:DBxxRxxmeet123@emeameetings.lync.com;gruu;opaque=app:conf:focus:id:1I323FSP>;tag=7390B0
Call-ID: 123456780a664c10ad6ccda4829aa0bf
CSeq: 1 INVITE
ms-diagnostics: 1009;reason="No match for domain in DNS SRV results";domain="emeameetings.lync.com";fqdn1="sipfed.resources.lync.com:5061";source="sip.company.com"
Server: RTC/6.0
ms-telemetry-id: 825AB686-9EEC-527A-BFF9-58128FC97449
ms-split-domain-info: ms-traffic-type=SplitFedIn;ms-remote-fqdn=sip.company.com

04/13/2018|15:42:01.330 1CEC:1CF0 INFO  :: End of Data Received -10.11.12.13:8080 (To Local Address: 10.21.22.23:56831) 1018 bytes

I do have some troubles to understand whose error this is. The source for the DNS error is company's SIP access pool. But on there I could query the SRV records like:

nslookup -q=SRV _sipfederationtls._tcp.emeameetings.lync.com.

without any troubles. So who does have the troubles? And which part is suffering about the timeout?

Because that user is in internal networks, the outbound connection goes via the proxy (see port 8080).