Error: Full Text Index Error after installing 4.0.5 HF4 "Value cannot be null." (4253031)

After installing 4.0.5 Hotfix 4. The event log is full of errors like:

Error performing message index (inner loop)
Exception Source: mscorlib
Exception Type: System.ArgumentNullException
Exception Message: Value cannot be null.
Parameter name: At least one element in the specified array was null.
Exception Target Site: WaitAny
Date and Time: 07/10/2008 10:54:51
Machine Name: Example
IP Address: 192.168.101.1
Current User: Example\ArchiveMgr_Service
CLR Version: 2.0.50727.1433
Platform (version): Win32NT (5.2.3790.131072)
Application Domain: Archive Full Text Index Service.exe
Assembly Codebase: file:///C:/Program Files/ArchiveManager/Archive Full Text Index Service.exe
Assembly Fullname: Archive Full Text Index Service, Version=4.0.5.132, Culture=neutral, PublicKeyToken=null
Assembly Version: 4.0.5.132


---- Stack Trace ----
at System.Threading.WaitHandle.WaitAny(WaitHandle[] waitHandles, Int32 millisecondsTimeout, Boolean exitContext)
at AfterMail.FullTextIndex.MessageIndexer.IndexMessages()