I tried to build this repo on branch mc1.14 with ./gradlew clean build
but I run into this

My expectations is to checkout and have a successful build.
Do you have any clue, what's wrong here ?

a ./gradlew --stopmade the job

Ok, on macOS ./gradlew --stop made it work.

But on Ubuntu 18.04 I run into

Does someone has a hint, how to get rid of it

Yes !
#8 helps to solve it