I can confirm that sophos on-demand scan cause the problem
for me the error message was (but the file which is not able to open was random):
dpkg: error processing archive /var/cache/apt/archives/nodejs_8.14.0-1nodesource1_amd64.deb (--install):
unable to open '/usr/lib/node_modules/npm/node_modules/tar/lib/mkdir.js.dpkg-new': Operation not permitted
I can confirm that sophos on-demand scan cause the problem apt/archives/ nodejs_ 8.14.0- 1nodesource1_ amd64.deb (--install): node_modules/ npm/node_ modules/ tar/lib/ mkdir.js. dpkg-new' : Operation not permitted
for me the error message was (but the file which is not able to open was random):
dpkg: error processing archive /var/cache/
unable to open '/usr/lib/