Click "Details" on the error box and copy all the information here, there's not enough information to help you currently.
 
here is detail


************** Exception Text **************
System.IndexOutOfRangeException: Index was outside the bounds of the array.
at Ultima.Art.IsValidStatic(Int32 index)
at FiddlerControls.ItemShow.worker_DoWork(Object sender, DoWorkEventArgs e)
at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
 
Looks like it's still not compatible with the latest client files.
You may need to replace Ultima.dll in the UOFiddler directory.
You can probably take Ultima.dll from the ServUO shard folder and put it in the UOFiddler folder.
 
Yes this looks correct -does this load for you with out errors? (at times having programs in the x86 folder tend to give a problem) but if fiddler now opens then your good :)
 
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