I had the same problem. My config was latest patchbox and latest modep.
Importantly (I think) is that I’d installed the plugins using the approach described here
The command
file /var/modep/lv2/calf.lv2/calf.so
reports
/var/modep/lv2/calf.lv2/calf.so: broken symbolic link to ../../../lib/arm-linux-gnueabihf/calf/calf.so
The symbolic link /var/modep/lv2/calf.lv2/calf.so has a relative path of …/…/…/lib/arm-linux-gnueabihf/calf/calf.so, which resolves to /var/lib/arm-linux-gnueabihf/calf/calf.so which does not exist.
I used the following to update the symbolic link (and update its ownership and permissions) to point to /usr/lib/arm-linux-gnueabihf/calf/calf.so
ln -sf /usr/lib/arm-linux-gnueabihf/calf/calf.so /var/modep/lv2/calf.lv2/calf.so
sudo chown modep:modep /var/modep/lv2/calf.lv2/calf.so
sudo chmod 775 /var/modep/lv2/calf.lv2/calf.so
and I can now add Calf plugins successfully 
I’m not sure if there might be any other plugins that have a similar issue.
S’manth x