Among other things solved by the August Cumulative Updates, here is an issue which arosed after installing this patch.
One of those issues is that if you modify a default list form (Editform.aspx, DisplayForm.aspx) using Sharepoint Designer 2007 and on this modified form you had an inpage user picker control, when you try to access this page in internet Explorer you will get the error from the title of this blogpost.
Type ‘Microsoft.SharePoint.SPFieldUserValueCollection’ in Assembly ‘Microsoft.SharePoint, Version=188.8.131.52, Culture=neutral, PublicKeyToken=71e9bce111e9429c’ is not marked as serializable.
The explanation for the message is, taken from http://msdn.microsoft.com/en-us/library/ms182350(VS.80).aspx ,”To be recognized by the common language runtime as serializable, types must be marked with the SerializableAttribute attribute even if the type uses a custom serialization routine through implementation of the ISerializable interface.“
The error appers only when using the Sharepoint Designer modified form.
The workaround for the behavior consists in replacing the modified forms wih the default ones, which might not be a feasible approach because you really need the customized templates.
A quick and easy fix is to remove the user control from the page and replace it with a label.This works fine if you are just DISPLAYING the control.But what can you do if you want to use the control in the editform and allow the user to search for a person ?
The easiest fix is: Install the December Cumulative Updates where this issue is addressed.