Problem solved ! I received two emails with links to verify my address. It was clicking the link in the first email that lead to the "banana error 500". when I discovered the second email and clicked the link in that one, the link worked and my email was verified. So the only error is in the error message: it should says something like "link expired" or something similar.

Same problem as Jade with incorrect link in the email confirmation email, leading to "banana error 500". Tried the chatbot which was rather useless in that it kept misunderstanding my query. "Talk to person" is not available anymore, but you can message the support team, which will come back to me "in one or two days"...  I'm using dozens of email change and verification processes on other sites at the moment and most of them are seamless. I am surprised that it doesn't work well on TradeMe.