Uable to find endpoint message


I am new in this Forum - but happy about the experience of many users here.
My question: I installed Jitsi in a “normal” way on a webserver with 12 cores, 64Gb Ram, 1Gb Network - no problem for our german school (“Gymnasium” with 630 users).

Then I tried zu change the prosody version to enable the “lobby function” - I became problems and switched back to the old version.

Since this time - or maybe earlier - we have problems like bandwith problems (black screens), the users dropped out the sessions and the jvb.log says very very often:

“EndpointMessageTransport.endpointMessage#586: Unable to find endpoint to send EndpointMessage to: a3c5cf0c

Now, we haver a maximum from 80 to 120 users in groups from 10 to 25 the same time - this is not too much, or ?

Do you have any suggestions ?
Is ist perhaps dependent on the fact, that I have no ipv6-Host ?

best regards,
Christoph Günschmann, Mönchengladbach, germany