If you are using the freeware Adobe Reader and facing the issue, sign out from Adobe Reader, reboot the machine and sign back in. Check for any pending updates of Adobe Reader from help>check for updates, reboot the machine after updating Adobe Reader and try converting the PDF file.
Have you tried converting other PDF files or is it specific to one PDF? is it possible to share the PDF file with us so that we can test it at our end?
is it a personal computer or an office computer? If the machine is connected to a managed network, try connecting the machine to a different network environment like a guest wireless network or a mobile hotspot and check.