Hi @mirek,
Welcome to the forum! 
I can confirm this behavior on Linux with Processing 4.1.1.
Reproducible command line example:
$ git clone https://github.com/FOSStriangulator/FOSStriangulator
$ processing-java --sketch=./FOSStriangulator/FOSStriangulator --run
FOSStriangulator.pde:0:0:0:0: Cannot find a class or type named “PApplet”
The issue is that the error message is not descriptive at all…
Can you try opening an issue on the Processing GitHub repository?