That error can happen if an account has been closed by Pinterest. However, sometimes it occurs just because Pinterest has been slow to respond. It's always a heart-stopping moment when you see that. Have you had an email from Pinterest? Try it a couple of times. If you still get it, use your proxy and login manually to your account to see if you can still get in.
No, I think if the email is ALWAYS opened in firefox with the proxy, you should be ok. But I know that gmail comes into my normal home IP and I look at it. I am just wondering whether any graphics in those Pinterest emails could be used to verify your location or IP address.
Reading the info on that (I don't know whether it HAS to include a graphic), it seems that piece of software can notify you of the reader's IP address, which, if they read in their normal browser could identify that they are in a different area from where they are "pinning".
I have my emails set automatically to text only. I don't allow images to be shown as they can be used to track whether an email was opened. I wonder whether Pinterest tracks which emails from them were opened and checks against the proxy? Or is that just paranoid?
Yes, there is a wordpress plugin called wpclone. I used that for one site and it worked well. It didn't work on a much bigger site. There are a couple of other plugins but I didn't know how to use them. One had a video "for beginners", but I couldn't make head nor tail of it. The beginners they had in mind must have lived on a different planet!