After upgrading to Qt 5.12.2 all clients got a disconnect after about 5 minutes of connection time. Reproducible with Qt 5.12.2 and Qt 5.12.3, downgrading to 5.12.1 solves this issue.
Not sure if this is a Qt bug or not, so I'm reporting it here first, however it looks to me like a bug in Qt.
Mumble/Murmur 1.3.0_rc1
Archlinux
Edit: Only the Qt version of murmur is relevant, client runs latest Qt just fine with murmur on Qt 5.12.1.