please check that the following file /etc/dhparam/dh-default.pem exists if you do not need to generate it or, for a faster solution, create a simlink to any of the existing ones, so create a symlink manually:
ln -sf /etc/dhparam/dh8192.pem /etc/dhparam/dh-default.pem
possibly lower (i.e., dh4096 or dh2048)