I too just downloaded the newest Paint.net 4.0.12 AND the latest psd plugin 2.5.0 . Now I can't save a freshly created file and it won't open any psd files that I've previously created and/or downloaded. I did the "2 stage copy" as stated above and my "FileTypes" folder does have the PhotoShop.dll file in there along with only one other dll (IcoCur.dll).
The file opening error is this:
System.TypeLoadException: Could not load type 'PaintDotNet.Threading.PrivateThreadPool' from assembly 'PaintDotNet.Core, Version=4.12.6099.39434, Culture=neutral, PublicKeyToken=null'.
at PaintDotNet.Data.PhotoshopFileType.PhotoshopFileType.OnLoad(Stream input)
at PaintDotNet.FileType.Load(Stream input) in D:\src\pdn\src\Data\FileType.cs:line 496
at PaintDotNet.Functional.Func.Eval[T1,TRet](Func`2 f, T1 arg1) in D:\src\pdn\src\Base\Functional\Func.cs:line 158
The save error is this:
Preview file size (error)
Then after clicking ok I get this error:
System.TypeLoadException: Could not load type 'PaintDotNet.Threading.PrivateThreadPool' from assembly 'PaintDotNet.Core, Version=4.12.6099.39434, Culture=neutral, PublicKeyToken=null'.
at PaintDotNet.Data.PhotoshopFileType.PhotoshopFileType.OnSave(Document input, Stream output, SaveConfigToken token, Surface scratchSurface, ProgressEventHandler callback)
at PaintDotNet.FileType.Save(Document input, Stream output, SaveConfigToken token, Surface scratchSurface, ProgressEventHandler callback, Boolean rememberToken) in D:\src\pdn\src\Data\FileType.cs:line 293
at PaintDotNet.Controls.DocumentWorkspace.<>c__DisplayClass240_0.<DoSave>b__2() in D:\src\pdn\src\PaintDotNet\Controls\DocumentWorkspace.cs:line 2925
at PaintDotNet.Functional.Func.Try(Action f) in D:\src\pdn\src\Base\Functional\Func.cs:line 174
Hopefully these errors will pinpoint the problem.