"Namespace prefix 'xsd' is not defined" (Compatibility between SharePoint 2010 and IE9)

As usually, a problem starts with the following screen:

Here is an error which fired while we were adding a new user to a SharePoint 2010 Security Group. What can we do?

As explain in a previous post, we have to dive deeply within the SharePoint logs to have more information about this "explicit" error message.

Following this guide, we found:

System.InvalidOperationException: Namespace prefix 'xsd' is not defined. at System.Xml.Serialization.XmlSerializationReader.ToXmlQualifiedName(String value, Boolean decodeName) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderDictionaryEntryArray.Read1_Object(Boolean isNullable, Boolean checkType) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderDictionaryEntryArray.Read2_DictionaryEntry(Boolean checkType) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderDictionaryEntryArray.Read3
ArrayOfDictionaryEntry() 7cd48c88-0fa8-4143-88b5-c311caecf34f

What can we learn about this user friendly message? SharePoint is just saying us that:

Namespace prefix 'xsd' is not defined. 

This well-known error message in SharePoint came from a compatibility issue between SharePoint 2010 and Internet Explorer 9. To bypass this problem, you can switch your browser in IE8 compatibility mode.

This can be done using the Developer Toolbar (F12 key while browsing). 

Inside this Developer toolbar, change the browser mode from "IE9 Standard" to "IE8 Standard".

Now your problem is gone. You can add users without any problems.

And if you do not want to do it every days, here is another solution. Add the following line in your master page to enforce Internet Explorer to use the "IE8 Standard" mode:

<meta http-equiv="X-UA-Compatible" content="IE=8"/>