ewitness api error

Hi we are using an in house install of the evidence api https:// [server] /evidence/api/v2/{guid}pdf/false https:// [server] /evidence/api/v2/{guid}/html/false while running some tests, we are seeing the following error, could you please help with info on how to resolve this error: {"data":"A PDF Document Engine command failed to execute successfully, producing the following error: \"DocEngine Message: requestId = \"4b65e87c-6e3e-48b1-aa19-714816676a02-56193-hqeslapt01c-corp-fin\", code = \"4\", msg = \"[GetElectronicEvidence] PDF engine unable to find converted documents from: /opt/DE/cache/20180621/4b65e87c-6e3e-48b1-aa19-714816676a02-56193-hqeslapt01c-corp-fin_0_1808610333_35105\"\".","status":"ERROR"} Thank you