Hi,
Can you see if below information helps and see how it goes for new reports received post below changes:
Can you please get in touch with your system admin and make sure that "sans serif font" is not missing
from the system because we have seen similar cases in past for unreadable format in past when font is installed in OS.Also as this is linux based machine can you make sure we install libfontconfig on OS and see how it goes on restart of services if required on new reports?
shell> sudo apt-get install libfontconfig