Error:

“Server Error in '/WebDAV' Application.
Could not load file or assembly 'Passageways.Modules.SystemPermissions' or
one of its dependencies. The system cannot find the file specified.”

If you encounter this error when trying to open documents, this usually means there are a few files on the server that need copied to another location.

To fix this, on the server you'll want to go to the location of your portal's bin folder, which should be in a location similar to:

Drive:\inetpub\wwwroot\**Portal Folder**\bin\

Here you'll want to find these two files:

Passageways.Modules.SystemPermissions.dll    and
System.Web.Optimization.dll

Select these files and choose to Copy them.

Then navigate to 

Drive:\inetpub\wwwroot\**Portal Folder**\WebDAV\bin\

and Paste those files to this folder.

Once that is done, you should be able to download or open documents properly again!