Could not load file or assembly 'CrystalDecisions.CrystalReports.Engine ...(Exception from HRESULT: 0x80131040)

Full error is Could not load file or assembly 'CrystalDecisions.CrystalReports.Engine, Version=13.0.4000.0, Culture=neutral, PublicKeyToken=692fbea5521e1304' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Trying to make modifications to an old ASP .Net project and hitting this error when my code tries to generate PDFs from XML data. It previously worked fine and I haven't actually changed that part of the code.

The bin\CrystalDecisions.CrystalReports.Engine.dll file is version 13.0.14.1720 as per Windows Explorer 'File version' details. Is this the 'assembly reference'? If so, what/where is the 'assembly's manifest'?