NOW it works! I didn't know that the complete PSFilterPdn folder had to placed in the effects folder, that was not part of the first Readme file. I placed only the two mentioned files and the MessagePack.dll there. My bad. I've downloaded the PSFilterPdn Plugin again and now I know that the whole folder is needed there.
That's interesting, because the Readme file of the Plugin doesn't mention this file, only these:
I've copied the messagepack.dll in the Effects folder and the error message is gone: BUT: The Plugin doesn't show my installed Plugins (Nik Collection).
I've moved all the files from the folder in the "Effects" folder of Paint.net. It worked the whole time, damn.
Edit: Now it works. I don't know why but I reinstalled it the third time and now everything seems to work. Thank you anyway!
I don't know why but it doesn't work anymore for me. After Avast deleted my current Nik Collection I had to reinstall it again together with the 8bf plugin.
When I want to start one of the filters I get this message.