Let me just add that we are using a custom tile provider which has worked out fine until this problem suddenly appeared. Let's assume that some of the tile requests gets HTTP 500 errors. Will this be handled? What about corrupt images?
The exception often occurs when all the visible tiles are loaded, so it may have something to do with preloading other zoom levels.
More exception info:
Message=No imaging component suitable to complete this operation was found.
at System.Windows.Media.Imaging.BitmapDecoder.SetupDecoderFromUriOrStream(Uri uri, Stream stream, BitmapCacheOption cacheOption, Guid& clsId, Boolean& isOriginalWritable, Stream& uriStream, UnmanagedMemoryStream& unmanagedMemoryStream, SafeFileHandle& safeFilehandle)
Message=Exception from HRESULT: 0x88982F50