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)
at PaintDotNet.FileType.Load(Stream input) in d:\src\pdn\paintdotnet\src\Data\FileType.cs:line 496
at PaintDotNet.Functional.Func.Eval[T1,TRet](Func`2 f, T1 arg1) in d:\src\pdn\paintdotnet\src\Base\Functional\Func.cs:line 158