and a funny Story setting my umask - long Story short:

1.) my old Approach

cat ~/.bashrc.d/set_umask

# ...
# umask 0077
# ...

works fine in Shell Context, but not in Desktop Context, e.g. for Dolphin

2.) ALL tried Solutions (/etc/login.defs, pam_umask.so etc.) DID NOT WORK - WHY - ANY IDEAS ?

3.) Finally, setting the desired umask in /etc/passwd WORKS:

sed -i 's/:\/home\/user:/umask=077:\/home\/user:/' /etc/passwd