Talk crashes randomly from inside lan to outside client

I am running NC15 on a home server on Debian 9. I have configured a coturn server.
When i make a video and / or audio call inside my Network on NC Talk, it runs fine. Also when i call someone outside of my Network from a device also outside of the LAN, it runs fine.
However, if i make a call from inside of the Network (where my server is running) to an outside client over the internet, it runs for a minute or two but then always crashes (black screen, no audio). This happened on the NC Talk Android app, and on Firefox on a Windows and on a GNU/Linux machine.
I am using a Fritz!Box Router with port forwarding for SSL and coturn. I noticed, that when i make a call from inside of the network to the outside, the network usage monitored on the server increases to almost 8 mbits (in and out). I have a suspicion that it crashes when the network usage peaks.
Should i make a Bug report on this, or is that something, that has a fix?