After upgrading an older version of Sitecore to version 10 you might see below error with the call to LinkManager.GetItemUrl:
[ArgumentOutOfRangeException: startIndex cannot be larger than length of string.
Parameter name: startIndex]
System.String.Substring(Int32 startIndex, Int32 length) +14342534
Sitecore.Links.UrlBuilders.Helpers.ItemPathBuilder.TryRemoveSubPath(String subPath, String& itemPath) +48
Sitecore.Links.UrlBuilders.Helpers.ItemPathBuilder.GetRelativePath(Item item, SiteInfo site) +205
Sitecore.Links.UrlBuilders.Helpers.ItemPathBuilder.Build(Item item, SiteInfo site) +105
Sitecore.Links.UrlBuilders.ItemUrlBuilder.UpdateItemPath(Item item, UrlBuildModel model, ItemUrlBuilderOptions options) +44
Sitecore.Links.UrlBuilders.ItemUrlBuilder.Build(Item item, ItemUrlBuilderOptions options) +246
The same code most likely ran without any issue in the older Sitecore version. Usually this is caused by the SiteDefinition and specifically the values for the rootPath and startItem attributes. Consider below configuration:
<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:security="http://www.sitecore.net/xmlconfig/security/">
<sitecore>
<sites>
<site name="website" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content/home" startItem="/" language="en" database="web" domain="extranet" />
</sites>
</sitecore>
</configuration>
In above code the startItem is set to “/”, and it turns out this will cause the above error. Changing the startItem to an actual item, and removing this from rootPath will fix the issue:
<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:security="http://www.sitecore.net/xmlconfig/security/">
<sitecore>
<sites>
<site name="website" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content" startItem="/home" language="en" database="web" domain="extranet" />
</sites>
</sitecore>
</configuration>
Jeroen's Sitecore Blog 