This problem is related to the source of the document. For example you are opening a document from Documentum server and the service on Documentum is problematic. Thus you are able to search and find the result, but you are not able to open/download the document. You can identify this problem by observing the URL (if the URL does not contains "-search", this mean that the service from the document source is having issue).
Contact SBS Support stating on which source of document you are having issue accessing with the URL of the document.
Example of the screen expected:
After trying to open the document to view
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