I have already had a working Master - Slave Configuration on my Jenkins, but an update broke it so I had to reinstall it (didn\'t have a backup yet).
But now after reins
Ok, I found it myself!
In the Global Security, I didn't have a TCP port for JNLP agents enabled, which automatically disables the line "Launch agent via Java Web Start"
As mentioned here:
Manage Jenkins => Configure Global Security => Enable security => TCP port for JNLP agents Set it to either Fixed (for this option also set the port number) or Random.