I can confirm that a clean install on a newly formatted device on a box (which was affected by this problem) runs fine now.
The used box is an older intel i7-2600 CPU (8 cores) with an also older Radeon R9 285/380.
The amdgpu module _is_ loaded as lsmod shows, but the installation itself and the update to 5.4.0-29-generic by the installer runs without any error by update-initramfs
I can confirm that a clean install on a newly formatted device on a box (which was affected by this problem) runs fine now.
The used box is an older intel i7-2600 CPU (8 cores) with an also older Radeon R9 285/380.
The amdgpu module _is_ loaded as lsmod shows, but the installation itself and the update to 5.4.0-29-generic by the installer runs without any error by update-initramfs