weird error that came out of now where

this is the error :

Error2"bin\Debug\OPC-UA Server1.vshost.exe.manifest;bin\Debug\Quickstarts.EmptyServer.vshost.exe.manifest;obj\x86\Release\OPC-UA Server1.exe.manifest" is an invalid value for the "InputManifest" parameter of the "GenerateApplicationManifest" task. Multiple items cannot be passed into a parameter of type "Microsoft.Build.Framework.ITaskItem".OPC-UA Server1

what does that mean ?