I'm running into the same issue on Mint 19.3:

Traceback (most recent call last):
  File "webtest.py", line 11, in <module>
    camera = pyfakewebcam.FakeWebcam('/dev/video20', 640, 480)
  File "/usr/local/lib/python3.6/dist-packages/pyfakewebcam-0.1.0-py3.6.egg/pyfakewebcam/pyfakewebcam.py", line 56, in __init__
    fcntl.ioctl(self._video_device, _v4l2.VIDIOC_S_FMT, self._settings)
OSError: [Errno 22] Invalid argument

its seems
Proposed solution was to install the kernel module from github instead of using the officially distributed v4l2-loopback package from Ubuntu 18.04

umlaeute/v4l2loopback#172

That seems to fix it, thanks!

Ubuntu 18.04
Linux 5.3.0-46-generic

#remove apt package
sudo modprobe -r v4l2loopback
sudo apt remove v4l2loopback-dkms

#install aux
sudo apt-get install linux-generic
sudo apt install dkms

#install v4l2loopback from the repository
https://github.com/umlaeute/v4l2loopback.git
cd v4l2loopback
make

#instal mod
sudo cp -R . /usr/src/v4l2loopback-1.1
sudo dkms add -m v4l2loopback -v 1.1
sudo dkms build -m v4l2loopback -v 1.1
sudo dkms install -m v4l2loopback -v 1.1
sudo reboot

Do you happen to know if these steps are relevant for Ubuntu 20.04? Thanks!

Using video2 instead of video1 worked for me. (Ubuntu 20.04)

I have the same problem with Ubuntu 21.04. (I have already tried all mentioned fixes). Any ideas?

I have the same problem with Ubuntu 21.04. (I have already tried all mentioned fixes). Any ideas?

Do sudo modprobe -r v4l2loopback

see the output of ls /dev/video*

In your modprobe command choose a virtual device number that is not already present.

In my case I used modprobe v4l2loopback devices=1 exclusive_caps=1 video_nr=7 card_label="v4l2loopback"

With 7 being an id that was not existing prior to activating v4l2loopback.

@JanSurft Thanks a lot! Now it works! I think the explicit phrase of the video ID did the trick.

sudo modprobe -r v4l2loopback
sudo modprobe v4l2loopback devices=1 exclusive_caps=1 video_nr=7 card_label="v4l2loopback"
ls /dev/video*

@JanSurft Thanks a lot! Now it works! I think the explicit phrase of the video ID did the trick.

sudo modprobe -r v4l2loopback
sudo modprobe v4l2loopback devices=1 exclusive_caps=1 video_nr=7 card_label="v4l2loopback"
ls /dev/video*

For me, it was /dev/video20. Why does that happen, and can I set the number manually to 2?