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

modprobe -r v4l2loopback
modprobe v4l2loopback exclusive_caps=1

This Fix does NOT WORK - no Cam in Chrome:

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