Inviting Participants failing.

So we have this scenario:

Support Person 1 is connecting to a customer - this is fine, they can see the customers screen etc

Support Person 1 invites Support Person 2 to also join the customer, Support person 2 gets the invite and clicks "Join" on the popup

Teamviewer then shows the yellow status light thing with "Authenticating" then it cancels itself.

I checked the logs and found:

2021/05/11 10:48:26.219 38208 30796 G1   CBlackWhitelistCheckRemote::Check: sent request {91f6c26f-0f5e-4943-8471-8079f78c2902}, put it in pending requests map
2021/05/11 10:48:26.277 38208 40152 G1   BlackWhitelistCheckResultHandler: black/whitelist check passed, continue with remote control invitation
2021/05/11 10:48:26.881  3924 24080 S0   CSendCommandToMaster::SendBCommandToMaster: CC=3 CT=71
2021/05/11 10:48:27.073 38208 30796 G1   VoIP: System performance (last 10s): CPU load of system = 7%
2021/05/11 10:48:27.084  3924 24936 S0   SecureNetworkConnection::SendCallbackHandler(): [ remoteID: XXX419XXX9 connection: XXX819XXX8 remoteConnection: XXX589XXX4 ], Error: SecureNetwork (Connection not found)
2021/05/11 10:48:27.084  3924 24936 S0   RetryHandle::HandleRetry(): Trying resend to 103 failed with error SecureNetwork:7, retrying (2 retries remaining) BCmd: CC=0 CT=0
2021/05/11 10:48:27.847 38208 42572 G1   VoIP: Sender: Audio pipeline: BasicVoiceCapturerDashIn #2: HalfDuplexSafeguardModule: EchoCancellationFilter: PowerIn: 0; Histogram (#samples=3000): avg=0.000000, Range = (-inf,inf), FF = A||; AEC quality: signal-reduction (avg) = 0.0dB; Histogram (#samples=3000): Range = [0,60)+=1, FF = A||............................................................ 

Connection IDs blanked with X's

Any ideas ? this has been working fine up until the last update (all clients on the latest teamviewer v15.17.7)