Uncaught exception: System.IndexOutOfRangeException

I'm using log4net 1.2.9 and got these uncaught exceptions thrown from
my application causing it to crash this past weekend:

System.IndexOutOfRangeException: Probable I/O race condition detected
while copying memory.

I am using log4net with an asp.net 1.1 web service, and am writing
entries to the log for every request. This is on a windows 2003
server, using iis6, and web gardening is turned on, so multiple
instances of w3wp.exe are running and processing requests.

Here is the full stack trace:
System.Web.Services.Protocols.SoapException: Server was unable to
process request. ---> System.IndexOutOfRangeException: Probable I/O
race condition detected while copying memory. The I/O package is not
thread safe by default. In multithreaded applications, a stream must
be accessed in a thread-safe way, such as a thread-safe wrapper
returned by TextReader's or TextWriter's Synchronized methods. This
also applies to classes like StreamWriter and StreamReader. at
System.Buffer.InternalBlockCopy(Array src, Int32 srcOffset, Array dst,
Int32 dstOffset, Int32 count) at System.IO.StreamWriter.Write(Char[]
buffer, Int32 index, Int32 count) at
System.IO.TextWriter.WriteLine(String value) at
System.IO.SyncTextWriter.WriteLine(String value) at
log4net.Util.LogLog.Error(String message, Exception exception) at
log4net.Util.AppenderAttachedImpl.AppendLoopOnAppenders(LoggingEvent
loggingEvent) at
log4net.Repository.Hierarchy.Logger.CallAppenders(LoggingEvent
loggingEvent) at log4net.Repository.Hierarchy.Logger.ForcedLog(Type
callerStackBoundaryDeclaringType, Level level, Object message,