I'm getting a similar error with 1.50 beta3. In my case, the fault that starts the chain is that I'm trying to open a secured PDF without providing a password or anything. That generates a PdfReaderException. The problem is that the PdfReaderException is trying to use the PdfSharp-gdi DLL's Resources\Messages.restext resources... only they don't exist. And it seems as though the reason they don't exist is pretty simple - the Build Action for both the Messages.restext and the Messages.de.restext is set to "None" in the source code I have. I changed that to "Embedded Resource" and rebuilt and tried again and got the PdfReaderException as expected, which I can handle cleanly.
Is there a process by which I can submit my suggested change? In this particular case it's probably not worth it
but if I find more things in the future that I could help address, I'd prefer to feed fixes back rather than having to maintain my own version of the codebase...