I have an opportunity card for the Museum of Mistakes. Clicking on it shows me the loading screen, but then drops me back to where I was before, instead of showing me the options on the card.
This sometimes happens to me (with just any card), and what usually works is reloading the page or closing my browser then opening it again.
That can happen when you still have a card for which you no longer have the unlock requirements. Did you drop watchful or something?
That was it - I had some items giving me Watchful boosts, and re-equipping them made the card work. Thanks!