Solved:
Somehow, the owner / group ownership got (?) changed for the folder /tmp/processing where tmp files are created for running a sketch. The owner/group was “root / root” and i’m not sure what the initial value was.
If I change those to my current user with
$ sudo chgrp GROUPNAME /tmp/proccessing
$ sudo chown OWNERNAME /tmp/proccessing
It works again.
Not sure how long though. and what I did that those ownerships got changed or what else might be amiss.
EDIT: calling “sudo …/processing-java --sketch=… --run” inside home/USER/.config/lxsession/LXDE-pi/autostart sets user/group of /tmp/processing root/root somehow