npm install causes errors like npm ERR! tar.unpack untar error on Debian

Installing Grunt as in the Bootstrap documentation shown, I first installed grunt-cli globally with npm install -g grunt-cli and now I'm trying to execute npm install, but only getting errors:

root@devvm:/var/www/axit/portfolio/public/bower_components/bootstrap# npm install
npm ERR! tar.unpack untar error /root/.npm/wrappy/1.0.1/package.tgz
npm ERR! tar.unpack untar error /root/.npm/wrappy/1.0.1/package.tgz
npm ERR! tar.unpack untar error /root/.npm/brace-expansion/1.1.0/package.tgz
npm ERR! tar.unpack untar error /root/.npm/delayed-stream/0.0.5/package.tgz
npm WARN optional dep failed, continuing form-data@0.1.4
npm ERR! tar.unpack untar error /root/.npm/is-property/1.0.2/package.tgz
npm WARN optional dep failed, continuing request@2.54.0
npm ERR! Linux 3.2.0-4-amd64
npm ERR! argv "node" "/usr/local/bin/npm" "install"
npm ERR! node v0.10.35
npm ERR! npm  v2.7.4
npm ERR! path /var/www/axit/portfolio/public/bower_components/bootstrap/node_modules/grunt-saucelabs/node_modules/sauce-tunnel/node_modules/request/node_modules/form-data/node_modules/combined-stream/node_modules/delayed-stream/test/integration/test-delayed-http-upload.js
npm ERR! code EPERM
npm ERR! errno 50

npm ERR! Error: EPERM, open '/var/www/axit/portfolio/public/bower_components/bootstrap/node_modules/grunt-saucelabs/node_modules/sauce-tunnel/node_modules/request/node_modules/form-data/node_modules/combined-stream/node_modules/delayed-stream/test/integration/test-delayed-http-upload.js'
npm ERR!  { [Error: EPERM, open '/var/www/axit/portfolio/public/bower_components/bootstrap/node_modules/grunt-saucelabs/node_modules/sauce-tunnel/node_modules/request/node_modules/form-data/node_modules/combined-stream/node_modules/delayed-stream/test/integration/test-delayed-http-upload.js']
npm ERR!   errno: 50,
npm ERR!   code: 'EPERM',
npm ERR!   path: '/var/www/axit/portfolio/public/bower_components/bootstrap/node_modules/grunt-saucelabs/node_modules/sauce-tunnel/node_modules/request/node_modules/form-data/node_modules/combined-stream/node_modules/delayed-stream/test/integration/test-delayed-http-upload.js' }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.
npm ERR! Linux 3.2.0-4-amd64
npm ERR! argv "node" "/usr/local/bin/npm" "install"
npm ERR! node v0.10.35
npm ERR! npm  v2.7.4
npm ERR! path npm-debug.log.ba707f2a7a688e388708bbe88e2dd4ed
npm ERR! code ETXTBSY
npm ERR! errno 62

npm ERR! ETXTBSY, rename 'npm-debug.log.ba707f2a7a688e388708bbe88e2dd4ed'
npm ERR! 
npm ERR! If you need help, you may report this error at:
npm ERR!     <http://ift.tt/1NicLxs;

npm ERR! Please include the following file with any support request:
npm ERR!     /var/www/axit/portfolio/public/bower_components/bootstrap/npm-debug.log

I've tried to delete node_modules and to rerun the npm install, but it hasn't helped.

The environment: VirtualBox with Debian guest system and Windows 7 host system.

I supposed shared folders permissions issues, since there were some problems with it, when I was installing Bower. But symlinks are enabled for this shared folder and I started the VM as admin.

What is causing thhese errors and how to solve this problem?