I found this in a log.

EPiServerInstall Verbose: 0 : Adding/Updating assembly redirect for EPiServer.Community.Webmail,
EPiServerInstall Verbose: 0 : Adding/Updating assembly redirect for EPiServer.Community.Webmail.XmlSerializers,
EPiServerInstall Verbose: 0 : Progress Status:      Activity:Installing  Description:Installing,
EPiServerInstall Verbose: 0 : Progress Status:      Activity:Installing  Description:Installing,
EPiServerInstall Warning: 0 : An error has occured and the transaction will be rolled back,
EPiServerInstall Warning: 0 : Rolledback,
EPiServerInstall Verbose: 0 : ,
EPiServerInstall Verbose: 0 : An unhandled error has occured:,
EPiServerInstall Verbose: 0 : Filename: ,
EPiServerInstall Verbose: 0 : Error: Cannot add duplicate collection entry of type 'virtualDirectory' with un,
EPiServerInstall Verbose: 0 : ique key attribute 'path' set to '/EPiServerCommunity/Modules/ImageGallery/Thum,
EPiServerInstall Verbose: 0 : bnails',
EPiServerInstall Verbose: 0 : ,
EPiServerInstall Verbose: 0 : ,
EPiServerInstall Verbose: 0 : When executing,
EPiServerInstall Verbose: 0 : ,
EPiServerInstall Verbose: 0 : At C:\Program Files (x86)\EPiServer\Community\4.0.517.255\Install\System Script,
EPiServerInstall Verbose: 0 : s\Install Site (No database).ps1:304 char:28,
EPiServerInstall Verbose: 0 : +     New-EPiWebVirtualDirectory <<<<  -SiteName $properties.Site.SiteName -Parent,
EPiServerInstall Verbose: 0 : Path $properties.Site.ApplicationName `,
EPiServerInstall Verbose: 0 : ,
EPiServerInstall Error: 0 : Error, System.Management.Automation.CmdletInvocationException: Filename:
Error: Cannot add duplicate collection entry of type 'virtualDirectory' with unique key attribute 'path' set to '/EPiServerCommunity/Modules/ImageGallery/Thumbnails'

 ---> System.Runtime.InteropServices.COMException (0x800700B7): Filename:
Error: Cannot add duplicate collection entry of type 'virtualDirectory' with unique key attribute 'path' set to '/EPiServerCommunity/Modules/ImageGallery/Thumbnails'


   at Microsoft.Web.Administration.Interop.IAppHostElementCollection.AddElement(IAppHostElement pElement, Int32 cPosition)
   at Microsoft.Web.Administration.ConfigurationElementCollectionBase`1.Add(T element)
   at Microsoft.Web.Administration.VirtualDirectoryCollection.Add(String path, String physicalPath)
   at EPiServer.Install.IIS7.Installers.NewWebVirtualDirectoryInstaller.Install(IDictionary stateSaver)
   at EPiServer.Install.InstallationManager.Install(Installer installer)
   at EPiServer.Install.WebServer.WebAppInstallationManager.CreateVirtualDirectory(String siteName, String parentPath, String virtualDirectoryName, String physicalPath, Boolean disableScriptExecution, String defaultDocuments)
   at EPiServer.Install.WebServer.Cmdlets.NewWebVirtualDirectoryCmdlet.ProcessRecord()
   at System.Management.Automation.Cmdlet.DoProcessRecord()
   at System.Management.Automation.CommandProcessor.ProcessRecord()
   --- End of inner exception stack trace ---
   at System.Management.Automation.Internal.PipelineProcessor.SynchronousExecuteEnumerate(Object input, Hashtable errorResults, Boolean enumerate)
   at System.Management.Automation.PipelineNode.Execute(Array input, Pipe outputPipe, ArrayList& resultList, ExecutionContext context)
   at System.Management.Automation.StatementListNode.ExecuteStatement(ParseTreeNode statement, Array input, Pipe outputPipe, ArrayList& resultList, ExecutionContext context)

put, Pipe outputPipe, ArrayList& resultList, ExecutionContext context)

Is this an IIS-related error?

Something was wrong with my IIS website, when I created a new one it worked.