The mojoPortal.Web project has a reference to the dll form the _libs folder and when you build it should pull the dependency from the _libs folder into the /bin, but every now and then there can be environment issues where it doesn't happen due to permissions or something and then you can fix it by manually copying the file from _libs folder if needed.
When I pull changes and apply updates and rebuild it works fine for me, but I have seen this kind of thing for various dlls once or twice in the past. I remember at one time this was happening for me with the log4net.dll way back when.
That file (Microsoft.Web.Preview.dll) has been the same version for a very long time so any copy from any recent version should be identical.
Best,
Joe