Hi,
Please, first try to recycle the application pool that is assigned to FileVista in IIS Manager.
If you still get the same error, then it seems the application can not write to C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files due to permission issues.
Normally this path is writable by the application pool accounts, as by default it has IIS_IUSRS group with Modify permission. Also if you do a permission change, recycle the application pool again so that the changes are recognized.