You must have an old PrismExplorer zip.
Is it possible you downloaded PE on or about the same day that we posted 5.2.2 (4 Sept I think)? We didn't get the updated copy of PrismExplorer in place until late Tues 8 Sept.
As I write this I'm staring at the web.config in the PrismExplorerWeb directory of the freshly downloaded zip and it is telling me:
<!-- ... Make sure that the Version is correct for your installation.
-->
<add assembly="IdeaBlade.EntityModel.Server, Version=5.2.2.0, Culture=neutral, PublicKeyToken=287b5094865421c0" />
Your image of the error says "Version=5.2.1.0"
The only safe thing to do at this point is erase your installation of PrismExplorer, download a new copy, and try again.