I solved it this way:
$ brew uninstall --force node
$ brew uninstall --force npm
after it
$ brew install node
which suggested me to overwrite simlinks
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink share/doc/node/gdbinit
Target /usr/local/share/doc/node/gdbinit
already exists. You may want to remove it:
rm '/usr/local/share/doc/node/gdbinit'
To force the link and overwrite all conflicting files:
brew link --overwrite node
after executing
$ brew link --overwrite node
everything worked again.