We are not sure, why you experience this bug all of the sudden with 3.9.3. I also couldn't reproduce it with Jitsi.
From looking at the code and at your stacktrace it seems, that your Jitsi client receives a ping from the server before you are authenticated. Why is still unclear to me.
Jitsi obviously does not support ping (XEP-199) and therefore returns an error, which is then cannot be handled and results in the error.
I've hopefully fixed it. Could you try a new nightly, when it's available!?
