I think I fixed this by going into the Forum Configuration -> Forum User Interface settings.

I changed the threads per page to 10 and posts per page to 5 and it made the problem go away.  I don't really understand why though.  I am digging in to figure it out but if someone has ideas I am open to hearing them.

Thanks,

M

I will jump on the train in any case.  I am seeing this same error on my site.  Here is the stack trace. 

InnerException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.ThrowHelper.ThrowArgumentOutOfRangeException
StackTrace:
Message: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource) at System.ThrowHelper.ThrowArgumentOutOfRangeException() at System.Collections.Generic.List`1.get_Item(Int32 index) at DotNetNuke.Modules.Forum.TopThreads.ViewTopThreads.BindData() at DotNetNuke.Modules.Forum.TopThreads.ViewTopThreads.Page_Load(Object sender, EventArgs e)
 

thanks matthais. I did not realize it was the top threads add on causing this.  good work.