Suspected Error:
Linux App is not downloading the Minecraft Launcher correctly.
Full Report:
As it seems, the Linux App fails to download all files.
After a fresh install under Ubuntu 18.04, the App had problems downloading the pack, after a restart it seemed to be fine (actually it did not download all files). It did not start, so checking the error log:
java.io.FileNotFoundException: /home/mudar/FTBA/bin/launcher_profiles.json (No such file or directory)
After some messing around I figured out that this file is created by the Minecraft launcher, copying a file from a Minecraft install did not work properly. So I figured I'd try to paste in the install of the Minecraft launcher. This worked to the point that I could log in, but it failed to start as it did not have all the files. Exiting and redownloading the pack worked to the point that I could see the forge install in the launcher.
temporary fix:
Download and install a Minecraft Launcher from the Mojang website and copy the folder into the FTBApp/bin/ directory.