Como encontrei uma resposta em Upwork Community Forum postado por Dmitriy K :
% bl0ck_qu0te%Cancelar pacotes guardados anteriormente:
apt-mark unhold libnss3 libnss3-nssdb
(para 64 bits)
mkdir -p /tmp/libnss-tmp
wget http://security.ubuntu.com/ubuntu/pool/main/n/nss/
libnss3_3.19.2-1ubuntu1_amd64.deb -O /tmp/libnss-tmp/libnss3_amd64.deb
dpkg -x /tmp/libnss-tmp/libnss3_amd64.deb /tmp/libnss-tmp
sudo cp -rf /tmp/libnss-tmp/usr/lib/x86_64-linux-gnu/* /usr/share/upwork
rm /tmp/libnss-tmp -rf
(para 32 bits)
mkdir -p /tmp/libnss-tmp
wget http://security.ubuntu.com/ubuntu/pool/main/n/nss
/libnss3_3.19.2-1ubuntu1_i386.deb -O /tmp/libnss-tmp/libnss3_i386.deb
dpkg -x /tmp/libnss-tmp/libnss3_i386.deb /tmp/libnss-tmp
sudo cp -rf /tmp/libnss-tmp/usr/lib/i386-linux-gnu/* /usr/share/upwork
rm /tmp/libnss-tmp -rf
A resposta para o Ubuntu 15.10 está aqui - Problema ao carregar o UPWORK