Hi, I have a site that wont run after upgrading. It's has no third party modules or customisation from the standard install (apart from skin). It's on 7.3.1, when I run the upgrade it completes fine but when I view the site I get this error:

[FormatException: Input string was not in a correct format.]
   System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) +10896279
   System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) +145
   System.String.System.IConvertible.ToInt32(IFormatProvider provider) +46
   System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider) +510
   DotNetNuke.Collections.CollectionExtensions.ConvertValue(Object value) +100
   DotNetNuke.Collections.CollectionExtensions.GetValueOrDefault(IDictionary dictionary, String key, T defaultValue, Func`2 converter) +64
   DotNetNuke.Collections.CollectionExtensions.GetValueOrDefault(IDictionary dictionary, String key, T defaultValue) +55
   DotNetNuke.Entities.Portals.PortalSettingsController.LoadPortalSettings(PortalSettings portalSettings) +923
   DotNetNuke.Entities.Portals.PortalSettings.BuildPortalSettings(Int32 tabId, PortalInfo portal) +27
   DotNetNuke.Entities.Portals.PortalSettings..ctor(Int32 tabId, PortalAliasInfo portalAliasInfo) +94
   DotNetNuke.Entities.Urls.AdvancedUrlRewriter.Handle404OrException(FriendlyUrlSettings settings, HttpContext context, Exception ex, UrlAction result, Boolean transfer, Boolean showDebug) +2136
   DotNetNuke.Entities.Urls.AdvancedUrlRewriter.ProcessRequest(HttpContext context, Uri requestUri, Boolean useFriendlyUrls, UrlAction result, FriendlyUrlSettings settings, Boolean allowSettingsChange, Guid parentTraceId) +5921
   DotNetNuke.Entities.Urls.AdvancedUrlRewriter.RewriteUrl(Object sender, EventArgs e) +531
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

Does anyone have any ideas? I've just complete about 8 site upgrades from various 7.x.x versions to 7.4.0 without any issues including version 7.3.1 but this 1 site is being stubborn.