permissions – ffmpeg can’t access file

I have seen this problem several times here or in other forums, usually user could not access the home folder. I have no problem with the home folder, ffmpeg works just fine there.
But in my folder on level / (/myFolder/) I can not make ffmpeg work.
So my command is:

ffmpeg -hwaccel cuvid -c:v h264_cuvid -i *.mkv -vf "hwdownload,format=nv12" -start_number 0 -vsync vfr -q:v 1 images/%07d.jpg

And the output is:

myVideo.mkv: No such file or directory

so it even resolves *.mkv to the actual video name. If I copy this video and the images folder to another location in my home folder and use the command there, it works like a charm.

For test purposes I already did several things which solved similar problems for other people

  • snap install ffmpeg –devmode (version n4.3.1)
  • chmod 777 -R /myFolder (I know it is a bad idea, this is a personal pc and I will change it again)
  • chown myuser -R /myFolder
  • went to the Ubuntu Software -> FFMPEG and activated all permissions
  • execute ffmpeg with sudo

The error message did not change in any kind of way.
I use Ubuntu 18.04 with ffmpeg n4.3.1 installed via snap, I can’t install it via apt since the version is too old and has no cuda support.

I am happy for every hint.