After updating to 4.8 I can no longer access my site. I get this error when I try:

Value cannot be null.
Parameter name: replacement

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: replacement

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[ArgumentNullException: Value cannot be null. Parameter name: replacement] System.Text.RegularExpressions.Regex.Replace(String input, String replacement, Int32 count, Int32 startat) +2321818 System.Text.RegularExpressions.Regex.Replace(String input, String replacement) +73 DotNetNuke.UI.Skins.SkinController.FormatSkinSrc(String SkinSrc, PortalSettings PortalSettings) +267 DotNetNuke.Entities.Portals.PortalSettings.GetPortalSettings(Int32 TabId, PortalAliasInfo objPortalAliasInfo) +3169 DotNetNuke.Common.Globals.GetHostPortalSettings() +258 DotNetNuke.Common.Globals.GetPortalSettings() +132 DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule.Init(HttpApplication application) +389 System.Web.HttpApplication.InitModulesCommon() +135 System.Web.HttpApplication.InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) +2600820 System.Web.HttpApplicationFactory.GetNormalApplicationInstance(HttpContext context) +347 System.Web.HttpApplicationFactory.GetApplicationInstance(HttpContext context) +139 System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +196

Does anyone have anny ideas?

What changes need to be made?

After upgrading from 4.5.5 to 4.8:

DotNetNuke Error: - Version 04.08.00 An error has occurred. An error has occurred.

Not terribly helpful, I'm afraid (to me or to anyone else).  Luckily it's my test site, so no harm done yet.