Evidently 4.0.6 broke it, as I'm getting this error after following the installation instructions very carefully, and the readme only specifies support for 4.0.5. I get this error when trying to import a .AGIF file.
System.OverflowException: Arithmetic operation resulted in an overflow.
at FileFormats.FileFormatGIF.GifReadExtBlock(Stream stream)
at FileFormats.FileFormatGIF.GifReadBlocks(Stream stream)
at FileFormats.FileFormatGIF.LoadFromStream(Stream stream, List`1& frames)
at PropertyBasedFileTypes.AgifInternal.Load(Stream input)