Users suddenly disconnect and can't connect again lib-jitsi-meet xmpp-websocket

Hi,
I use lib-jitsi-meet to implement my own frontend for jitsi-meet. I use xmpp-websocket for connections. I have a main jitsi-meet server and an additional jvb server.
The problem is, some users can’t connect, some other disconnect suddenly and can’t connect again, and some other’s can connect but they don’t recive audio/video.
This has happened scince we switched to Xmpp-Websocket for serviceUrl.
I checked those user’s console logs, and found these:

<Object.r.strophe.log> Strophe: user connection callback caused an exception: TypeError: cannot read property 'substr' of null

<Object.r.strophe.log> Strophe: TypeError: Cannot read property 'isFocus' of undefined
at ie.onIncomingCall....

But other users have no problem at all.
how can I fix this?
Thanks in advance.