This occurs it is because you do not have the correct certificates installed in your Document Templates folders.
The certificate files are stored here:
These files are usually issued with a new software release. Either you have not upgraded or your document templates have not been copied as part of the upgrade. The certificates files may have also been issued outside of a software release and issued as a separate maintenance release.
The current certificate files are attached to this ticket. To fix this issue unzip the files into your document management root folder and unzip.
Contact support to seek confirmation that your software version number is correct. How do I know what version I have installed?