Browser shows error "This page isn’t working" with "HTTP ERROR 500" when adding a new eBay account or updating an existing eBay account?

Problem description:
I'm adding a new eBay ID to SoldEazy, clicked "Add" button, entered eBay ID and shop initial, clicked "Save", then page jumped to eBay, I logged in and clicked "Agree", but only to find the browser showing page error like this

Answer:
When you were typing your eBay account username, there might have been some misspelling or some other mistakes, because this page only pops up when you are trying to grant access to an eBay ID that is different from the one you entered in SoldEazy.