I did a quick check and it seems to work fine for me. I assume you board is a shareable board. Has Martin got the invite to join the board and did he accept?
Also I believe (not sure though) that you can’t have guests from inside your own organization. Is Martin invited with an external email address (not within your own domain)?
Can you please try to open your account in an incognito window, or a different browser, and let us know if the issue persists?
If it does, can you please pop me over a private message with your email address and account URL and we can continue to look into this together