@Zedeau ,
Sorry for the late reply, but was to bussy with other things.
I worked out a this solution in order to pxe-install ubunto 20.04, but this error does not affect the installation.
As I’m not a developer, I have no idea from where this comes, but I will make some time to find out why it refers to this location.
Initially I extracted the ISO in this path “installation_media/ubuntu”, but it should not be necessary as the installation should be done using the ISO-file.
As @upeter metioned, you need to apply the method in the “Preseed default PXELinux Autoinstall” PXELinux template.
Today I was able to autoinstall ubuntu 22.04 as well with my workaround, and even without having an extract of the ISO for this version the installation, I get the same message in the apache log, but the installation went also well.
Only problem with ubuntu 22.04 is that the subscription-manager packages from atix ends up in an error (dependency for python3 version which is higher in 22.04).
I already informed @maximilian about this issue, so it is just a matter of waiting till they provide a new repo for ubuntu 22.04.