The Camera starts only once. Version 0.12.5.-1ubuntu1 is broken. Remove the module, then it starts again:

modprobe -r v4l2loopback
modprobe v4l2loopback

A better fix is to install v4l2loopback-dkms_0.12.5-1_all.deb from Debian.

modprobe -r v4l2loopback
dpkg -i v4l2loopback-dkms_0.12.5-1_all.deb
apt-mark hold v4l2loopback-dkms

Thanks to: https://github.com/obsproject/obs-studio/issues/4808